The conference wins. The organization's reputation with the hotel improves because their "attendees" used more rooms. If they were having trouble reaching their guarantee, you just put them one room closer. (Since most conference participants check into the hotel before they go to the conference registration desk, not having conference credentials won't be an issue. Most conferences have at least a few walk-in registrations, too, so not being on the pre-registered list won't be a problem either.)

The hotel wins. They sell a room at a profit, Maybe it's less of a profit than they could have made otherwise, but the room probably would have been empty otherwise.

The theoretical hotel you might have stayed at instead loses. Maybe they'll get more aggressive in going after conference business next time.

On balance, no less ethical than booking on Priceline.