Clemson receivers at the line of scrimmage all night long, which is legal, because they are allowed to bump the receiver there. If a DB does that when the offense is running a rub play, there can be no penalty for an illegal pick because the DB engaged in the contact. If the Alabama DB didn't engage the receiver as they had been doing all night, then it would have been an illegal pick.

Defenses often thrive by that lock-down strategy (ala BYU/Michigan), but as we saw last night, a defense can get burned by it as well.