They've already shown a pretty strong interest in the hand and think they are calling anyway. The final cap by you shows the last player in the hand is willing to "Cold Cap" (Thats a pretty strong sign of a good hand,unless I'm a maniac). At least by waiting you may get bet into on the turn by the 3bettor. If you cap I think its gonna be check to you on the turn.

If I'm raising the flop its for value not really to protect. I'm just willing to give up a little here in hopes to protect against draws on the turn and punish inferior made hands.