Ele trabalha para a união should be accepted as well!!:)
Why can't I say "ele trabalha pelo sindicato" here?
trabalhar para o sindicato means he works for it, but "trabalhar pelo sindicato" would mean "by means of".
I'm sorry, I'm wondering if you can clarify and explain a bit more the difference between por/para o and pelo/pela? This is something I've been wondering for a little while...