False Start of Illegal Shift

[Ia-Ref]

Illegal shift is a foul at the snap. Just like a player in motion going forward. Live ball foul.
You cannot asume the player will not get set before the snap or if two are in motion that they both will not set.

The play should be allowed to continue because:
B could foul on the play (double foul, reply), A could lose yardage so the penalty would be declined, A could have a more serious foul (multiple foul - B choice), A could fumble or throw an interception.