Actually at peak times such as holidays, Disney
does restrict guests on the beach during fireworks to "only" those that are staying there or have an ADR at a full service restuarant.
This is checked at the gate and even at the boat dock and monorail and on the beach and is enforced.
So no; the beach is not actually open to everyone all the time, it is however not enforced during non peak times.
At one time there were signs posted on the beach at the Polynesian, stating the beach was for the exclusive use of resort guests. It blew down during one of the hurricanes and has not been replaced.
I think everyone could agree that this is not problem, unless it is over crowding the facilities. I don't think anyone regardless of where you are staying would want to be crowded out of enjoying your own resort by guests not staying there.
I agree the polite thing to do as a guest would be to stand until, right before the show begins and then take any available seats. But politeness seems to be out lately.