I've read of hotels that have sent out letters like this to all guests during times when special events policies are in place without realizing that some guests booked before the policies were instituted. Clearly, that should never happen. But, humans sometimes make mistakes.
Call the hotel during normal business hours and talk to someone in the on-site Reservations office. This may be a simple clerical error.
If it is not an error, then you should certainly fight it.