But they can form one, or sort of. At least fighting this messed up system as individual won't work. I read 60% of all h1b visa applications and granted visa originate from 2 single Indian job distribution centers. I don't want to know how much cash and non cash favors are exchanged between those shops, their lobbyists and vested interests in DC.
That's another benefit of an H1B visa (from the perspective of the employer) that I did not consider earlier. Imagine someone on an H1B visa contemplating joining a union. If they get fired, they got 3 months to line up another sponsor. Few people would join a union and take that risk. So H1B's also help prevent the workforce from unionizing.