no pass interference on punts?

[PackersFTW]

the rams lined up to punt, but the punter passed the ball instead, and one of the bears clearly knocked down the receiver before the ball got there. they said there is no flag for pass interference because in a punt formation, it is legal to knock down the receiver. how can this be? if this is true, they need to get the information out to punt coverage teams that if they fake it, just wait for the ball to go in the air and then just tackle the receiver. this is one of those rules that not many know about i think.