Ah, so if it's in big enough lettering, it's not a scam.

If I ate at a sandwich place every day for a year, and then they put on their credit card receipt that you sign, "$100 charge if you don't eat your entire sandwich within our shop", then it's okay?

It's a silly rule, that of course has little basis in fairness. Many US locations have similar things, but I don't think anywhere near $50.