Personally i don't think Chambers should of been binned. He didnt deliberately hold the player down for an extended time, he actually let go of the player fairly quickly, but because of the attacking player flopping about and dropping the ball the ref gave one of those 50/50 penalties. Chambers may have accidentally given away a penalty, but i don't think he deliberately gave away a penalty (which is what a professional foul is. It has nothing to do with try scoring chances). I've seen a few times that players give away penalties while defending their try line, they'll essentially lay on an Opponent, with no effort to move, until a penalty is given. Now because their isn't a try scoring chances obvious they don't go to the bin, but they should as giving away a deliberate penalty is exactly what the professional foul sin bin is for.