I've never been a mod but I guess people do it because they like it, or they like how it makes them see themselves. They might join a protest for the same reasons. Being demoted and becoming a nobody, on the other hand, doesn't sound like much fun.
The powerjannies are not good people. Their reputations are earned. They care, but only to the extent that it never puts their tiny bit over power they have over other people in danger. They are at least 50% of the reason reddit has sucked for a really long time.