They made the right call picking the flag up. If you watch the play all the way through, and from the angle coming from behind Pettigrew, you see that Pettigrew grabbed Hitchins facemask before the contact that would have otherwise been called defensive pass interference. There's no way that can be pass interference on the defense, if it is then the rule is terrible. Mike Pereira dropped the ball trying to explain the play, I have to think he just didn't watch the play all the way through (to see the facemask) before he reached his conclusion.
If anything, the penalty should have gone against Detroit. It wasn't an intentional facemask I don't think, but it can't be the case that the receiver gets to pull the defender in by the facemask (or even just briefly grasp the facemask) and then the defender gets flagged for the contact that follows.
EDIT: Hitchens not Hutchins
It was PI because the defender didn't try and play the ball. The rules says if the defender tries to inhibit the receiver without trying to play the ball. Plus, the defender grabbed the jersey earlier in the play which should have been holding. I agree on the face mask but, it's rare they make that call. The other two, not playing the ball, has been called for years and the hold, as slight as it was, was a big point of emphasis this season.
The problem begins with the mixed crew and how they handled the call, shocking fans, for and against, with that late of a pick up. Happens but, rarely. The mixed crew thing gives the impression, well, what it is, poor team work simply because they have not been working together. Do mixed crews at the pro bowl. Not the playoffs.
The BEST call would have been offsetting penalties, a face mask on the receiver and a hold and/or PI on the defender. That would have been a MUCH better call and, I think, a crew that worked together would have come to that conclusion and quickly. Even if not a face mask, it's still a push off, penalty on the offense. Offset that with your choice of PI or a hold. Either way, it was a terrible call at a terrible time and had an enormous impact on the game. Detroit was seizing momentum and sapping Dallas's will by making several tough 3rd down conversions in a row. That is CRITICAL in football. This isn't to say Detroit doesn't turn it over on the very next play or that they score and end the game. It is to say that, if at all possible, it should be decided on the field and the NFL has done a great job of that with the use of off setting penalties, play the down over.
A crew that worked together, in my view, would have said 'offsetting' and it would be acceptable by every fans come Monday regardless of who won. We'd be talking about a great finish, however it played out, and not a call. That is worst case; talking about a call.