The unions negotiate fair indutry-wide rates and conditions for the members, not taxes.

Think of it like people having talked about their salary in advance so when they walk into a negotiation and hear a number that's lower than their peers, they can call bullshit and make a take it or leave it counter.

Companies get upset about unions because they're not paying fair pay right now and know it, and if everyone in a trade demands the same minimum rate then they have to accept it.