#1 - saying it is like the way we pay taxes even though there is a competing incentive there to not want to pay implies there is more incentive to cheat on mafia than to not cheat.
If you assume there is more incentive to NOT cheat than to cheat, then the problem goes away.
People who want to win at all costs are probably not suited to mafia. At the same time, taking that chance to cheat puts at risk your entire future possibility of playing and winning unless you find a new group to play with.
If you then look at it from the point of view of someone who wishes to continue playing mafia and enjoys it, and cares what people think of them, cheating becomes a very inferior option. The win/lose of cheating/not cheating doesn't pay out. Even if you are solely focused on winning and don't care what people think of you, you will spend your life having to find new marks to cheat. For what? Not for reputation...
#2 - Let's say you think "well people can cheat and get away with it" Yes they can. But have you thought further on?
Let's say you know who everybody is. Now you cannot use any of that information unless you can come up with a way that you could have known it. If you're too smart, game after game, it will become obvious. So now you have to spend all your "game energy" not on working out what role is what but on working out legitimate ways that you could have worked out this or that. Legitimate reasons to goodie kill a baddie. Legitimate reasons to tell everyone to move their votes. Legitimate reasons to convince the goodies they need to block player A and "DO NOT LYNCH THAT GUY!" (omg he is the role spy!)
You can't lynch a baddie by yourself and you can't kill a baddie until it is clear to more than yourself that they are bad. At least you can't get lucky N1 kills on a regular basis, maybe once or twice in a mafia career. You can't constantly kill off the save or the role spy N1 and then claim their roles without spying them.
To the proposed situation, if one person asked another to tell their role and the 2nd guy said "hey this guy cheated!" and the first guy said "BS, that's just his strategy to win the game." (Btw, how would that even be a strategy? Why would you want to get a friend banned from a website ANYHOW? Even for a person who wants to win all the time, I don't see the strategy for it. There will be other players that are hard to beat. There will be other players who can step into the mafia role that guy just stepped out of.)
But in that situation, it is easy. "You two know each other IRL and are playing mindf***s so
and why did you bring that buffoon into our site, anyhow?"
In conclusion, for people whose main incentive is the fun you get in working out all the roles, cheating destroys that. For those who just want to win, you can't win if you can't keep playing and where is the achievement in cheating. The only positive would be to improve one's reputation but it would take a lot of work not to come undone and those who aren't sociopathic would be quite upset when all those that look up to them eventually see their true colours. (In my opinion.) Cheating ruins the cheaters enjoyment of the game or their ability to enjoy their win.
Polishing 30 MVPs for cheating is like the people who pay 100s of dollars to buy little icons in games. It is a picture that you did nothing to earn. How is that even worthwhile?