I don't know why, but I've never had any, um, gastrointestinal issues. I have heard that some other people have, though, and so I've told my not-used-to-homebrew friends to not drink a sixer at a time due to possible ill effects.
Maybe it's something that your body gets accustomed to? My husband and I both drink copious amounts of homebrew and never had a problem. But we did start with one or two at first. My husband even drinks the yeasty stuff at the bottom if there is still some beer in there.
Broken Leg Brewery
Giving beer a leg to stand on since 2006