> They want two adults [...] to rent separate single bedrooms

Then why do they even offer twin rooms?

Doorless bathrooms are not explained by saying hotels (that offer twin rooms) secretly want all twin room guests to pay double and use two single rooms. Most hotels don't even have very many single rooms!

It may be different in small places where there's only one or two hotels. But most cities have dozens to hundreds of hotels, not all owned by the same conglomerates. There is no way they'd miss out the entire twin-room market by pretending not to have them. And they certainly wouldn't take the reputational damage by pretending you'll get a normal twin room when you book, but hit you with a doorless bathroom twin room when you arrive. The booking website will have photos showing the typical room layout, in order to give prospective customers clarity about what they'd be booking, so they choose to book there.

I'm no hotel tycoon either, but the idea that it's a secret ruse to get people to pay double doesn't make any sense to me. The idea that they're blindly following some design trend, or that it lets them make the rooms physically smaller by giving the impression of a bigger room, are much more plausible ideas.