It's because at the speed of things during the game, if a ref clearly doesn't see something he'll go on precedent...kinda like our legal system.
If he seems a collision from Bruce Bowen and Kevin Martin...wtf is Kevin Martin...Bruce Bowen is a DPOY candidate...probably Martin's fault.
A lot of teams play slacker D all regular season...the refs get used to it...then all of a sudden play hard D in the playoffs and get burned. Ref sees Antoine Walker reaching...he hasn't seen Walker play D since 2004....he calls a foul.
Same thing why Jordan got all those calls. He started out his career attacking the basket and got pummelled, pretty much every single time. Detroit would usually send 3 guys to foul instead of just 1. After a few years of this the refs just called it in Jordan's favor. They're thinking "maybe it wasn't a foul, but 80% of the time it is so that's how i'm calling it."