Face guarding is basically an act of waving your arms or hands above the shoulders to block the view of a potential pass receiver while the ball is in the air. NCAA rules on pass interference require the face guarding to have contact to be a foul. No contact, no foul by NCAA rules.
The problem here is that there was clearly contact and the referee (or second referee) calling that contact, essentially, insufficient isn't something that is allowed under the rules. Now if a member of the offense had shoved the defender into the wide receiver, different story.
Anyway, for those currently curious about the play and rammifications, ESPN has an excellent argument about the potential fouls on that play, not all of them on the defender. It's found
here.
Ultimately, what might calm everyone a little is this snippet examining the impact of that play on the game's conclusion:
Is it the primary reason the Cowboys won and the Lions lost? No. The decision reduced the Lions' win probability at that point from 78 percent to a still-healthy 66 percent, according to ESPN Stats & Information.