Ok, so if feces gets into your bloodstream you get a nasty infection.
Hemorrhoids tend to bleed and occur in close proximity to a reliable source of feces.
Why don’t hemorrhoid sufferers die from septicemia?
Bleeding out is a surprisingly effective way of keeping bacteria from getting in. I suppose if someone with actively bleeding hemorrhoids managed to stuff fecal matter into the wound, something could get started.
Here is an explanation from one of the physicians here.
http://boards.straightdope.com/sdmb/showpost.php?p=14744554&postcount=2