In my experience, they usually take the counter-dad joke in stride, and we move on (sometimes they do make an obviously exaggerated expression as part of the joke). I’m probably an outlier, but I’ve always found “that means it’s free” quaint if just really trite; it’s just trying to be friendly and make my monotonous day a little more fun, and I understand from their perspective that it isn’t conspicuously overused. So I take the joke for its intent (I’ve never seen it used seriously, and imagining a remotely sane human being doing so strains credulity) instead of its actual novelty or cleverness. I will never make it because it’s so worn-out and I know it’ll make most people in retail groan, but I don’t begrudge people who do, since I’ve never seen it used in a sincerely harassing, negative way.