By restricting microtransactions to cosmetic elements, the thing they play on is one's vanity. Players who are content exploring the gameplay can do so unencumbered. There is no "special club" for players who buy items. They don't beat you more easily in the games. The main focus is still on winning the matches.
There is no incentive to buy those cosmetic items except players thinking "I want to look cool". Whatever Valve may do, that's not a vice they are responsible for creating, or that anyone is ever exempt from (we all have to buy clothes). They merely brought it, opt-in, to their games.