Rams' Johnson got away with blatant facemask on Patriots' Blount

Can anyone explain why Los Angeles Rams defensive back Trumaine Johnson wasn't flagged for this obvious facemask on New England Patriots running back LeGarrette Blount?

Sure, the play was over, but Johnson very obviously grabbed Blount by the facemask and tried to slam his face into the ground.

Missed and blown calls have become an every-week occurrence in the NFL this season. It's probably time for the league to make its game officials full-time employees.