And can someone explain to me why many Americans think that unions is a bad thing? (Or why unions are a bad thing in the US)
Depends on the union, but they've gone from being perceived as being beneficial to the working man to any combination of weak and ineffective, leeching off the hard work of individuals, bureaucratic systems that just create more obstacles to overcome, old boy networks that protect the old guard and/or provide preferential and protectionist treatment to people who abuse the system and their power in it. I mean, I could keep going on, but you get the idea. Regardless, whether or not that's the reality of the situation, that's how they're often viewed.