Yeah I totally agree with the call but given the Lions struggles on 3rd and long it wasn't a slam dunk decision. If they don't take the penalty chances are the Rams are trying the FG to take the lead, so worst case scenario is you're down 2 with 4 minutes to go so you atleast have a chance to drive down the field and win the game.
By taking the penalty you run the risk of them converting and then potentially killing all or most of the clock before kicking the go ahead FG leaving you with no chance to respond.