are you saying its transactional then? like a social contract of "it's my birthday, so I'm paying for my guests food and drink." You, my guest, have accepted that contract by bringing a gift?
This flies in the face of birthdays I'm used to. There's no expectation that If I invite someone to my birthday that a) they need to give me a gift (I would never expect that) or b) I'm paying for their food and drink. I guess because that social contract isn't in place, the idea that someone can come over and say happy birthday isn't a big deal. It's just a gathering that happens to be on my birthday.