Well they probably would not have a permit to sell anything except human food, so the point is moot. But yes, people will pull stuff like that, the only thing they promised was a print. Also they did not say you could pick which picture to print, so they can pick the one where you blink, etc. Portrait studios are famous for doing this kind of thing,,,come in for a $10 family photo, dress up the family, they snap 10 pictures (because the first one the kids are not still,....the second the baby cries,...the third I blink...) but of course the first snap is the $10 one, other pictures are only sold in 'packages', starting at $50. Happens all the time.
We have purchased groupon discounts to restaurants (and others like from restaurant dot com) and even though there are no restrictions on it, the establishment manager will put restrictions on it table side. I can understand things like liquor not being included in deals (even though the groupon never said it was excluded), but they will line item the menu when you get there, so be sure to show it before ordering. And yes, when they do that, we are inclined to not go back, and that is why they needed to drum up business in the first place.