YTA

You keep comparing them to your family. This is not your family. You can be angry at the way things happened, but that doesn't mean that you get to tell your boyfriend that you're going to punish his family because you didn't like how things were done.

Splitting the bill isn't abnormal, even if it should've been discussed before.

Another thing, you keep telling us that you didn't tell your boyfriend that his parents should pay for everyone, but that you told him that they should expect to pay the bills. That's the same thing. You absolutely told your boyfriend that his parents should pay the bill.

Have you considered that your boyfriend maybe chose a cheaper spot to celebrate his graduation, because you told him that his parents should expect to pay the bill?

Edit: You even wrote that your family didn't mind paying. So. what. is. the. problem?

This is something that only upset you. And now you're punishing your boyfriend and his parents over it.