Every snap is a live play. No where in the rulebook does it say that teams "cant get physical" if one team declares taking a knee.
Team B has a legit argument to wanting to play it out. As a general rule, "taking a knee" is a minimum loss of 1 yard when the QB takes a snap under center. In this situation, Team A has to make the effort to get out of the EZ, and has no right to complain about Team B's level of aggression to stop them from advancing the ball.
Would you have a USC if the QB just completed a long pass and was running up to the line indicating he would "clock" or "spike" the ball, and then ran a play instead?
If Team B is lulled to sleep and doesnt attack in either situation, that is their own fault imho. No foul.