For the
most part, they don't flag incidental facemask to facemask contact. Had SJB kept his face forward and wrapped up, he wouldn't have been flagged. And ironically, it may have been a better result as the Purdue receiver may have held on to the ball for a four-yard loss.
ARTICLE 3. No player shall target and initiate contact against an opponentwith the crown (top) of his helmet. When in question, it is a foul. (Rule 9-6)
Accepting that this was a penalty by the definition of the rule is not the same as agreeing that the rule is a good one. Personally, I don't like the rule. But SJB ducked his head and went for the big hit. That puts you at risk for the targeting call.
There are valid concerns about how this rule is written, whether or not it should even be in place, and the inconsistency among various conferences in enforcing it. But it is there for now, and the refs made the proper call based on the wording of it.