Perk gets it so often in the post because he's the only player on the team who consistently operates and attempts to score in the post.
In order to have a balanced offense, you need to have a down-low scoring threat and threats from outside. If the Celtics don't establish Perk down low, they don't really have any other options because Garnett and Wallace hang out 15-20 feet away from the basket. It's really difficult to "establish" Perk down there, though, because he's not a top scoring option. He's at his best when he's an afterthought on offense, for the Celtics and the other team - that's how he can get good looks.
So the problem isn't Perk, really, but rather that the team has nobody else who can do what Perk does (and even Perk doesn't really do it).