While you can freely use the transportation system, that does not carry over in to all the amenities of the resort. Only guests of each resort can use that specific pool. So if you are staying at the Contemporary, you can only use the Contemporary pool, not the Polynesian. The last time I stayed at the Beach Club we all had to wear wrist bands to show that we were registered guests of the Beach Club and had the right to use the pool. Those without bands were nicely invited to leave. People pay a lot of money to enjoy all the features of their resort and Disney protects those features for guests use only.