"Not causing people to make choices they will regret" is a pretty simple ethical principle.

Actually, it's contradictory. If they actually have autonomy, then you can't truly "cause" them to make a particular choice. So choosing to "not cause" them to make a choice is actually admitting they're not autonomous.

Ergo, given the definition of "objectifying" in use here, you are objectifying someone merely by trying not to influence them.