Unions are part of the market like anything else. If wages are higher, they aren't above market-clearing levels, those are the new market-clearing levels. If workers form a union and bargain collectively, that is what the market naturally set.

Do you apply the same argument for employers? Companies contribute to low wages. By collectively bargaining with employees (e.g. hiring at the local grocery store is centralized, you can't go around to all the individual managers and start a bidding war) they can negotiate wages that are lower than what the market would naturally set.