Personally, I suspect these letters are the thing of Urban Legend. Talked about often, but never seen.
If Disney was going to start dealing with commercial renters, common sense would dictate that those folks who have created websites for the express purpose of renting
DVC would be the first to go down.
It would simply take one day, one court order, filed by Disney legal to temporarly enjoin the violator from advertising and renting. Then the renter would have to prepare to fight the court battle.
This has not happened, as the sites are still up and running.