Well, usually people will want to play MP with those with a similar mindset. If 9 players out of 10 have decided on the type of game they want to play, and 1 is a troll who wants to ruin their game enters, there's clearly a conflict of interest. That's why people usually specify what sort of game it is they are hoping to play when they host a new game.
After all, competitive people find the most fun in playing competitively. To ruin the fun of another player is not a nice thing. If everyone else is in agreement as to what is the way to play they want to play, and have gathered around for the express purpose to play the game like that and have publicly announced it, it's incredibly rude to join their game and spoil it by playing in a way that's counter to that. It's equally rude for a super minmaxer to enter a game that's casual and leisurely, and just crush everyone with his unbeatable skill and abusing tactics.
In short, it's nice to play by the same rules as everyone else.