I was up in arms with the same thing while at the game so had to look up the rule. We are both correct.
ARTICLE 9. ROUGHING THE PASSER.
(e) A rushing defender is prohibited from forcibly hitting in the knee area or below a passer who has one or both feet on the ground, even if the initial contact is above the knee. It is not a foul if the defender is blocked (or fouled) into the passer and has no opportunity to avoid him.
It's obvious that many officials don't know the rules or just turn a blind eye to them.