Why are the handles on axes with single-edge heads curved? The axes with double-edge heads sure aren’t…
Also: Does anyone use adzes anymore?
I can’t answer the first question, though my suspicion is that it is for leverage. I can tell you that it is obvious that the handle on a two-headed ax wouldn’t be curved. Which way would you curve it? 
In answer to your second question, yes. Adzes are still used, though I suspect the vast majority of them are put to use for decorative effect rather than their original purpose. By this I mean to say that most adzes in use today are being used to distress timbers to make them look old rather than to hew timbers out of logs. On the other hand there are people like Roy Underhill.
I hope that makes sense.
The curved handle gives a slightly more comfortable grip. It’s similar to the reason a pistol has a grip that’s at an angle to the barrel, rather than in line with the barrel: That’s the way our hands are built. Although with an axe, the curve of the handle also keeps the handle from slipping through your fingers as easily, since the handle isn’t exactly in line with the centrifugal pseudo-force.