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.