This is done, presumably, so people dont buddy up to save money on rooms. They'll be too shy or modest. So if its 4 people traveling, say 2 couples, they'll rent two rooms instead of one.

Then why offer twin beds?

There are two possibilities:

They find this out in advance and find a 'normal' hotel and book a room there.

They find this out when they get to the hotel, complain to everyone, write bad reviews, never come back.

Lose-lose for the hotel (unless it's some remote destination with just one hotel there, but those are rare)

No doubt this same hotel is proud of their green credentials.

I can not even imagine sharing a room with another couple under any circumstances