This continues to not make any sense.
In most hotel pricings I've seen, twin rooms and double rooms cost the same. In fact, in the cheaper hotels, double rooms are just twin rooms with the beds bolted together (very annoying if you're a couple seeking romance). The hotel can reconfigure the rooms to match demand, as the only difference is whether the beds are joined.
As a random example (I don't endorse it, I just picked a random London hotel) https://www.booking.com/hotel/gb/crowne-plaza-london-ealing.... has "Standard Room" (choice of twin or double bed), "Standard Twin Room", "Standard Queen Room with Bathtub" and "Standard Queen Room with Walk-In Shower" options all at exactly the same price. Each option makes abundantly clear what type of bed(s) you get, and how many people can use the room.
Hotels that want to rent rooms to couples simply remove twin rooms from the list of rooms available. Only offer the double-bed option. People looking for a twin room go to the next hotel in the list. They don't need some secret plan to disappoint twin-room guests by not having a bathroom door so their next booking is two single rooms.
You and the OP both said "single" rooms. Is this key to unlocking the mystery? In my experience, single rooms literally have one single bed. Why are multiple people hoping to stay in one? Also from what I've seen, "single" rooms are more expensive than twin/double rooms, not just because you can't share the costs but because they literally cost more, because there are so few such rooms in the hotel. The hotel couldn't accomodate people if it compelled twin room guests to get two single rooms, it'd run out of single rooms in a jiffy and be left with a lot of twin/double room capacity. Most of the rooms are double/twin.
Why would any group of people book a single room? Is there some secret trick where multiple people turn up and bring their own beds with them, only to be foiled by a missing toilet door?