Can a hotel charge you an extra night when you're not even booked for that night?

I got a notice today from a hotel that they are charging me an extra night at $102 + tax for the 23 of May. However, I booked the room for the 27th and the 28th and that's it. I reported it to the Better Business Bureau to see if this was against the law. I had reserved a room for a convention at the convention rate this letter I received went out to all con goers who reserved a room under the special rate. Is this legal in the state of North Carolina or the U.S. because I don't see why we have to pay for a room 4 days in advance from our actually booked date.