I think a lot depends on what kind of wedding reception it was going to be. At the weddings in my family, we've tended to reserve a block of hotel rooms (even though we mostly live in town), and continue the party at the hotel after the reception. I did that for our wedding. Our reception was over at 11 PM, but we all stayed up and just hung out together until the wee hours of the morning.
Also, as a pp said.. usually when a block of rooms is reserved, they're close together. For ours, we had one whole wing (about 10 rooms) of a really small hotel. It was really nice, because everyone in that wing was part of our wedding party so we didn't have to worry about noise too much, or encountering strangers in the hallway.. actually we mostly left our hotel room doors open and just wandered from room to room.
In our case, I would have been very angry at the hotel, but also irritated at the guest, for two reasons: One, they declined the "afterparty" just to save money on their room. That is their right, so I'd really be more hurt than angry at that one. Second, though, they allowed the hotel to give a room which I had specifically blocked for our wedding party, to a stranger. I don't think that would be their right, since I had arranged to block the rooms.
I'm going against the grain here, I do think that you should have checked with the HC before agreeing to give up your room.