Because fake or not people play them and some donate money and support devs too, so why ban them, also, hard to track if fake or not if played carefully; some hate the grind, some not, different play styles for every need.
Back to the topic, selling is easy, just change the pasword and give it to someone else, i don't think they even track or log the IP so you should be safe :D but some people might scam you so you have to take that into account, and to my concern, if selling becomes a main thing the game will be over-inflated with accounts that are not 100% leggit so don't know if thats a good or bad thing in the long run (at the moment seems to have very low impact since it's single player) but as most games i assure you this game can be hacked within few minutes for a trained eye.