I'd say thats one big advantage to unionization and the political leverage that it can bring. Not sure it helps in California where you have a blue state. Getting rid of the cheap exploitative labor that H1's bring in and restoring the H1's original design which was to bring in the best and the brightest or the most skilled.

Why can't H1B employees leave? Can't they go back to their countries?

It’s hard to leave when your children have already made American friends, when they would suffer immensely going to a country they have no familiarity with. The disruption of money and emotional safety of moving to another country with no employment prospects is a horror I don’t want anyone to live through.

Just because you aren't literally a slave doesn't mean you have to choose between two options you don't want instead of trying to get the third better option.