> Why would any group of people book a single room?

To save money.

> 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?

Beds? Probably not. But, people (especially younger people, can sleep on the floor with climate appropriate (which, depending on the season and available heating, can be "none") coverings for warmth; I did this happily a fair amount in various groups aroun high school age, but I certainly wouldn't want to now in middle age.

> To save money.

If they want to save money, hostels are usually half the price of hotels. Why would they even choose a hotel in the first place?

Plus, my experience is that hotels will simply cancel your booking, or force you to upgrade, if multiple people turn up to check in for a single room. They don't need some passive-aggressive doorless bathroom, they have the right to tell you to book a 2-person room (whether twin or double bed) for 2 people.