If you think that is bad, wait until we talk about the infield fly rule. 
Or why a ball hitting the foul pole is fair, and a home run…
okay I’ll give you a new tack, and the one I thought of when I saw the OP.
With two outs in the first inning and Greg Maddux pitching, Mark Grace hits a grounder to Chipper Jones, who boots it, Grace reaching on the error. Sosa follows with a home run, and in fact so do the next six batters. All eight runs are unearned. I think this is asinine.
Of course the logic is that you “reconstruct the inning,” but what this fails to reckon is that Maddux would have had to face Sosa leading off the second inning, and it is certainly possible, even likely that Sosa would still have homered. Sosa’s run was by no means the result of Jones’ error, it was the result of a fat pitch, and Maddux deserves to have his ERA reflect that.
By the way, almost 10% of all runs are unearned. And since they are occasionally distributed unevenly, it is not unusual for a guy with a 4.00 ERA to have actually given up less runs than a guy with a 3.80.
The logic here (and I believe it to be correct), is this:
If the inning ends, the game undergoes a radical shift: Sosa goes to play an inning in the field, Maddux gets to rest in the dugout, talk strategy with the manager/pitching coach, etc. The game situation is dramatically different at the beginning of the next inning. Who knows? Maybe the Braves score five runs, if they don’t get discouraged by Sosa’s homer, Maddux gets energized by that, and strikes out Sosa in three pitches. Either way, the pitcher is not penalized by poor fielding, and they do keep statistics on total runs allowed.
IMHO, Earned run average is kind of dubious anyway. Just stick to the run average (RA). It is much more intuitive and makes more sense. Besides, the same leaders in ERA are certain to be on the same spots when their RA is computed. Bill James covers that very well with his sabermetrics.
They are?!???!?!!!?!?!?
Lets take a look at all the pitchers who have pitched 70 or more innings this year (sorted by Run Average) where ERAP is is position in the EAR rankings and RAP is his position in the RA ranking-
Name ERA RA ERAP RAP Dif
Martinez,P............ 1.44 1.61 1 1 0
Johnson,R............. 1.57 1.91 2 2 0
Brown,K............... 2.31 2.54 3 3 0
Person,R.............. 3.17 3.27 7 4 3
Maddux,G.............. 2.91 3.27 4 5 1
Leiter,A.............. 3.16 3.59 6 6 0
Meche,G............... 3.49 3.60 12 7 5
Pavano,C.............. 3.06 3.71 5 8 3
Benson,K.............. 3.19 3.71 8 9 1
Dempster,R............ 3.35 3.76 9 10 1
Wells,D............... 3.43 3.82 10 11 1
Glavine,T............. 3.68 3.83 14 12 2
Rusch,G............... 3.96 3.96 24 13 11
Neagle,D.............. 3.86 4.04 18 14 4
Baldwin,J............. 3.88 4.06 19 15 4
Wolf,R................ 3.83 4.08 16 16 0
Estes,S............... 3.81 4.13 15 17 2
Mussina,M............. 3.99 4.14 28 18 10
Ankiel,R.............. 3.44 4.17 11 19 8
Eldred,C.............. 3.91 4.19 20 20 0
Stephenson,G.......... 3.92 4.19 21 21 0
Vazquez,J............. 4.05 4.23 31 22 9
Hampton,M............. 3.68 4.27 13 23 10
Finley,C.............. 3.99 4.32 26 24 2
Park,C................ 4.17 4.34 36 25 11
Abbott,P.............. 4.23 4.35 37 26 11
Heredia,G............. 3.93 4.37 22 27 5
Radke,B............... 4.07 4.38 32 28 4
Benes,A............... 4.40 4.40 46 29 17
Colon,B............... 4.42 4.42 25 31 6
Lieber,J.............. 4.01 4.44 30 32 2
Ritchie,T............. 4.10 4.46 33 33 0
Stottlemyre,T......... 4.24 4.48 40 34 6
Rogers,K.............. 4.11 4.51 34 35 1
Castillo,F............ 4.23 4.54 39 36 3
Weaver,J.............. 4.26 4.54 42 37 5
Rueter,K.............. 4.23 4.56 38 38 0
Hudson,T.............. 4.30 4.58 43 39 4
Sele,A................ 4.32 4.59 44 40 4
Schourek,P............ 3.96 4.60 23 41 18
Reynolds,S............ 3.99 4.66 27 42 15
Anderson,B............ 4.50 4.67 50 43 7
Redman,M.............. 4.56 4.68 52 44 8
Parque,J.............. 3.86 4.71 17 45 28
Schilling,C........... 4.83 4.83 61 46 15
Trachsel,S............ 4.49 4.84 49 47 2
Kile,D................ 4.51 4.84 51 48 3
Pettitte,A............ 4.40 4.91 45 49 4
Reed,R................ 4.76 4.96 58 50 8
Tapani,K.............. 4.73 4.97 55 51 4
Haynes,J.............. 4.63 4.99 53 52 1
Hernandez,O........... 4.65 5.04 54 53 1
Millwood,K............ 4.85 5.11 62 54 8
Nomo,H................ 5.16 5.16 71 55 16
Ponson,S.............. 4.75 5.17 56 56 0
Appier,K.............. 4.24 5.17 41 57 16
Yoshii,M.............. 5.08 5.18 69 58 11
Hernandez,L........... 4.41 5.21 47 59 12
Helling,R............. 4.14 5.22 35 60 25
Bere,J................ 5.03 5.23 67 61 6
Milton,E.............. 4.81 5.25 60 62 2
Astacio,P............. 5.04 5.27 68 63 5
Sirotka,M............. 4.01 5.31 29 64 35
Suppan,J.............. 5.33 5.42 81 65 16
Holt,C................ 5.28 5.45 79 66 13
Penny,B............... 4.81 5.47 59 67 8
Loaiza,E.............. 5.19 5.48 74 68 6
Halama,J.............. 4.98 5.53 66 69 3
Perez,C............... 5.25 5.57 76 70 6
Bell,R................ 5.28 5.64 78 71 7
Escobar,K............. 5.30 5.65 80 72 8
Carpenter,C........... 5.49 5.67 85 73 12
Cordova,F............. 4.91 5.69 63 74 11
Bottenfield,K......... 5.60 5.70 88 75 13
Mulder,M.............. 5.27 5.72 77 76 1
Burba,D............... 5.72 5.72 92 77 15
Clement,M............. 5.24 5.77 75 78 3
Schoeneweis,S......... 5.08 5.78 70 79 9
Dotel,O............... 5.38 5.79 83 80 3
Rapp,P................ 5.18 5.79 73 81 8
Meadows,B............. 5.17 5.79 72 82 10
Mays,J................ 5.35 5.80 82 83 1
Hentgen,P............. 5.54 5.83 87 84 3
Mulholland,T.......... 5.70 5.88 91 85 6
Hermanson,D........... 4.94 5.93 64 86 22
Reynoso,A............. 5.69 5.93 90 87 3
Downs,S............... 5.43 6.06 84 88 4
Clemens,R............. 4.76 6.09 57 89 32
Dreifort,D............ 4.94 6.10 65 90 25
Villone,R............. 5.92 6.13 94 91 3
Martinez,R............ 5.52 6.13 86 92 6
Sanchez,J............. 5.85 6.14 93 93 0
Parris,S.............. 5.67 6.30 89 94 5
Cone,D................ 6.40 6.61 99 95 4
Yan,E................. 6.59 6.90 101 96 5
Mlicki,D.............. 6.33 6.93 97 97 0
Wells,K............... 6.03 7.12 95 98 3
Oliver,D.............. 6.66 7.15 104 99 5
Arrojo,R.............. 6.31 7.19 96 100 4
Olivares,O............ 6.64 7.20 103 101 2
Ashby,A............... 6.36 7.30 98 102 4
Elarton,S............. 6.43 7.31 100 103 3
Woodard,S............. 6.61 7.51 102 104 2
Ortiz,R............... 6.93 7.53 106 105 1
Lima,J................ 7.10 7.66 107 106 1
Daal,O................ 6.83 7.95 105 107 2
Yes, the top three are the same, but the average pitcher had a change, either + or - of 6.75 positions. ERA may not be perfect, but using a run average would penalize pitchers on poor fielding teams, ask almost any Chicago White Sox pitcher. Mike Sirotka’s ERA is 4,01 but his RA is 5.31 and he would fall 35 places if RA were used insted of ERA. Roger Clemens doesn’t fare mauch better (a 32 position drop)