That's a really obscure example. I have a lot of family around me and we have many weddings. Most of the time the venue is the caterer since that is what they charge for the venue.(500-900 people weddings, over 10 a year). Not always though.

BUT STILL, weddings are a really bad example here.

The people getting married get to chose from whom they want the table and chair covers, flowers, singer/band, catering, belly dancers/entertainment, lights people, AV people.

Those are choices you got to make and the bills will obviously come separate since there is clear distinction of entities. Heck, the venue and the singers might not even of said a word to each other, let alone have a billing agreement.