I’m not sure why it wasn’t treated as a seperate penalty. But - on the next play Matthews had an illegal use of the hands and it would have been first down regardless:
From the web:
“Packers LB Clay Matthews with a sack of Seahawks QB Russell Wilson in the third quarter!
A personal foul on the Seahawks was declined — with a loss of 15 on the sack.
On third-and-19, Russell Wilson completed a 29-yard pass down the field to WR Doug Baldwin. An “illegal use of hands” penalty was called on Clay Matthews — and it was first-and-10 for the Seahawks.”
Who’s to say that the play involving the hands to the face would have happened if the 15 yard penalty had been tacked on. The distance to go for first down would have been altered significantly, likely altering the play selection.