There is nothing wrong with replacing out of a mafia game. At all. It happens in every game, and people have learned to play around it. Well, ideally. Staying in and not playing is much, much worse.
Please refrain from criticizing people until you learn more about the game. Like, more as in what the average non Newbie Game newbie knows or more. Please and thank you.
Also you don't get to /in private games, by the way. My /in/in/in/in/in was meant to express my excitement because I've wanted a Gigabots Mafia for literally like 1.5-2 years (likewise with glyph), not really the same with random people. Otherwise you're just queuejumping. I learned that myself once and ended up not running the game because I felt like too much of a douche having gotten the game filled in such a manner. Laundry gets to hand-select his game's roster.
Just sayin just sayin.