If they were monsters, they wouldn't have accepted a sudden unwanted burden into their home.
Having a teenager suddenly dropped on your doorstep unexpectedly is a big deal. Your day to day life will have to change as a result of it, and it makes sense that they may want some compensation. There is also the possibility that they didn't care about the money at all and just wanted the kid to learn to make a living and develop a good work ethic or maybe they just wanted them to spend more time out of the house.
Either way, grossly ignorant to call people you don't know at all "monsters" just because you are unhappy they charged rent for having someone live with them.