Did the defender look back at the ball/play the ball? If not did the defender make any contact with the receiver while the ball as in the air?
One can argue the fairness of the point but one thing is certain (something every Pats fan should know from the 2011,2010 secondary), don't look back/play the ball, open yourself up for minor contact being PI. Every player knows this. Every player knows that if you don't look back you open yourself wide for a PI call. Many a situation like this in 2010-2011 occurred where Pats defenders weren't/wouldn't look back (something causing this board to steam) and getting called for frustrating/seemingly unfair PI because of minor contact.
Calling PI like this may not be fair but it the way it is. Don't turn around = very minor contact is now open to be called PI. Again the Patriots were victims of this many times thru the darker secondary days of a a few years ago.