@Omasan0
The price difference is based on averages.
We can't stop someone from sleeping in all the beds, but normally guests don't do this. Maybe someone snores or is sick, and the other one wants to sleep separately from them, and that is fine. But usually they don't go and jump on all three beds just to mess with me.
Two towels per person is what I provide, and sometimes they use them, but sometimes they don't. The water usage for two people showering with two towels each is different than four people showering with one towel each, and plus there is the mess and demands on the heating/cooling system of 4 people vs 2.
For example, when there are 4 people there is a much higher likelihood that one of them will have different temperature requirements, so I might have the situation of the A/C blasting while one person puts on the space heater near their bed.
With a larger number of people, there is also a higher likelihood that someone will ruin the sheets or comforter with some kind of mess: nosebleeds, or greasy makeup, or some kind of god-knows-what on the sheets (this has happened three times, always with groups of 4 or more).
My price for 2 people is based on the average amount of mess, cleanup, and water usage for 2 people. My price for 4 people is based on the average for 4 people. You can't pick out different individual things and talk about the price for this and that. I have had a party of 2 use up several rolls of toilet paper, while another group uses hardly any. Some guests eat all the snacks and some eat none. I just go off averages for groups of 2 and 4. For some reason, groups larger than 2 are much messier and use up more stuff... on average.
So you might ask, why should MY group pay more just because some other group was messy? There's no way for a host to know ahead of time who is going to be messy and who isn't, so we just have to go off averages.