Fifteen Million Merits takes place in a prison.
Though there is a lot of debate about the crossover worlds within Black Mirror, there is some distinct cross over between a few episodes. Namely, Fifteen Million Merits and several episodes after. The main song plays a central theme in many episodes following, and the in universe TV show, Hot Shot, is even shown on billboards and tv commercials in later episodes. One possible explanation is that the cells in this episode are actually a prison. Mind you, this is Black Mirror world, so any number of things could land you jail time. Pirating music, being late on your bills, being overtly rude to someone, minor sexual harassment, just about anything you could imagine being overly scrutinized could potentially wind you up in jail. Rather than punish them with harsh, overly stressful environments, they put them away in a cell far out into the country where they work for their power, food, entertainment, and even hygiene amenities. If you want a better life, you dance like a monkey and hope you become a pop idol and not a sex toy. Even the famous don't get let out and are instead just given better life styles. If we as viewers are to believe Bing's cell at the end does look outside, then there is no apocalypse happening and humanity would be just fine living outside where the oxygen is breathable enough that trees and birds can survive. It may very well be that the outside world exists and these people are just criminals used as entertainment fodder.- Your theory has been emboldened by Crocodile, where Bing's incident occurring in the Hot Shots competition is referenced. They may also simply be an underclass of slave labor who have known nothing else, which would make sense given that nobody has any thoughts of escaping.
- Or you could combine the two ideas: Denote those that have broken the law into slave labour. If they aren't rich of course. In-universe, this would likely be seen as a more "humane" method of sending criminals to a "proper" jail- or even Death Row.
