Judgement calls on the part of the ref.
The ref can always tell if both feet got down. Whether they would have come down after apush is a judgement call.
Like the tuck rule, the NFL prefers to have consistency in calls above all else.
I think you guys are over-reacting to this force out stuff.
For one thing, the defense has had every single friggin rule change go against them for the last five years. About time they got one go their way. Second thing is that it will not be as easy as everyone thinks to push a guy out. The DB can't push before the ball gets there, and then has to push before the receiver gets his feet down. Pretty small window.
The only plays that will be affected are the heave the ball to the outside and either the reciever comes down with it or hope for a PI call. In this case, the teams that will benefit most from the rule are those with shorter DBs.
This rule really favors the Pats.