Most of it also stems from the fact that once a guy is in mid dunk the chances of blocking are much lower then the chances of getting a foul. The worst thing you can do is make a 100% shot worse by fouling the player and giving a potential 3 point play.
See, I can understand this. Or being inside the restricted area. Or being in foul trouble and being handcuffed from defending like you want to. But I see an awful lot of players giving up on plays VERY early when guys go for dunks. It seems lie a lot of times when a guy gets a lot of steam and still has a stride to go before he starts to elevate for a dunk, you see guys already starting to get out of the way. That annoys the shit out of me. You don't even know if you can get there in time to make a play yet, but you're already moving away because you "don't want to be in the poster." That's kinda disgraceful to the game.