I don't think people are talking about big parties; rather, a table of people out for a small dinner. Obviously no restaurant is going to have a problem with a patron bringing in a cake when they're shelling out for dinner and drinks for 50!
Ahhh, I actually was thinking of large parties, that's where I have seen it. I guess it would be different for just a small, few people family dinner. Also home baked goods vs. bakery (made in a commercial kitchen) and the keeping a home made cake in the restraunt kitchen.
Now I think I'm over thinking this whole thing lol