Does it say in your contract that they will take it back? Mine doesn't. I knew that at one time they would allow some memberships to be returned so that they had inventory (memberships) to sell. The last newsletter indicated that they had plenty and that if members wanted additional weeks, they were available for just about the MF.
Due to the economy and general health of the resort business, they probably have an abundance and don't need more.
I knew they would cancel a membership for a price, but was led to believe that this was a courtesy that wasn't necessarily permanent. Perhaps they will get short on weeks to sell again and take more back. Who knows?
Anyway, to answer your question, it's only illegal if the practice is contrary to what is stated in your contact.
On the other hand, MROP membership has no underlying deed. The only 'hammer' that the BOD has to enforce payment of MFs is that delinquent members don't get to use or exchange their weeks if not paid up. That's probably the desired result of not paying the MF anyway.
Jim Ricks