... if I'd been given $40 worth of free food, you can bet my server would get at the very least a $10 tip. If the total bill was $80 before the discount, said server would get at least $16 (20%) and probably more. Especially if that $40 discount wasn't a result of a problem or mistake, but was because the restaurant was buying food for all active or retired military today to honor them. If I got free food just for showing up, it'd inspire me to be generous.
And it did work that way for some people; most notably I had a couple who thought I was so great (mostly because I didn't rein in my sarcasm when the guy started giving me crap, I think) they told the manager how awesome I was. It was just one group of younger guys who made me grumpy.