Roderick: Every tour I've ever done, either as an opener or as a headliner, there's been a clause in the contract that says the headliner has a right to see the opener's butt(s). It's just one of those bits of music business boilerplate that keeps making it into contracts, even though most people don't remember it's there and almost no one ever invokes the clause. It's a good idea to read every contract thoroughly.
Still, I would chalk it up to experience and consider yourself now a fully initiated member of the Guild of Openers, an organization that only admits artists who have worked their way up in the touring game according to the strict traditions of our profession. When you start doing major headlining tours you may find that a butt-peepage clause is something you want retain.