Well, it is actually nice to know that some folks like your husband are considerate of others, even at their own expense. Nice if more folks were like that on many matters.
However, I do side with you. It is a dump station. You've paid your camping fee and are entitled to use it. The park decided to erect their tent their and have Yogi make an appearance. They could have also put up a sign that said "Dump station closed from XX AM to XX AM, sorry for the inconvenience" but they didn't.
Maybe they were just trying to answer that eternal question for the kids: "Does a (Yogi) Bear **** in the woods?" :)