Yeah... the thing is that, there is some amount of cost / reward calculation going on here. You want to be able to demonstrate a threat to Negan's slaves, but at the same time not expend so much ammunition.
Given the fact that this is a television show, they obviously went very heavy on the "more firepower now please" side of the equation, which is unfortunate, but not really a matter except for the purist.
Yes also you are right about not just taking a pot shot at Negan when they had the chance. He's going to be in a few more episodes.