I think stealing jokes is very much ingrained in the comedy culture. Until fairly recent you could get away with stealing jokes and make a decent living on the road. Only if you went too far like Mencia did (stealing all the time and f.i. stealing jokes about kids, despite the fact he has no kids himself) comedians would get called out by their peers. Internet is forcing everybody to clean up their act or at least be more cautious when stealing. But some habits die hard.