Just to clarify, you are not allowed to just be topless on the Island... but I highly recommended going to the island to try AN... it's such a good time. So relaxing, comfortable, peaceful, and all around good time.
Just to clarify what pier I'm talking about, it's the main one pictured below:
I think the reasoning is simple. If you are on the island, you need to be AN, I would assume that some people would feel uneasy if everybody wasn't held to the same standards there. And on the main beach, I suppose it could cause some issues with gawkers walking back and forth or something like that. This way, if you want to wear clothing, you are on the beach, if you want topless, you go out to the pier, if you want to be in the buff, head on over to the island. There is a wonderful spot for whatever you desire! I think it's all about making everybody feel comfortable with whatever they want to do and to limit the amount of drama that could take place.