The problem isn't so much that unions are incentivized to bargain for as much as possible. Every one who participates in the economy is trying to get the best deal for themselves. The problem is that politicians aren't restrained by cost. If a union demands that hole diggers be paid $100 an hour instead of $10 an hour, a politician will gladly give it to them even if it will bankrupt the state.

Edit: I think I should note that I'm specifically referring to unionized government workers.