I wouldn't say Wario treats his pals that badly, aside from refusing to pay them. In-game you can see them all go clubbing together, and during the Potluck and Dance Party in Gold they all wonder where Wario is. As if it's a strange occurrence for him not to show up to these kinds of things.
Then in the Developer Diaries it's shown they do all sorts of stuff together; ranging from picnics to game nights at 9-Volt's.
So yeah, while Wario may be a heel he certainly has appeal. Otherwise his friends wouldn't stick around and offer to make games for him time and time again.