For one thing, if you get laid off you only have two weeks to find another job or you need to leave the country. This balance of power is lop sided so the employer can get away with abusing the employee unreported.
OK,got this part - this is referring to abusing the employees.
I read the former comment like that they are abusing/missusing the "technical Greencard mechanics" to get their employees past some checks/approvals/permissions/whatever or acquire whatever "special benefits" by working around whatever rules
Which also "conveniently" means those employees can effectively never unionise.
No, green cards actually remove that constraint. You are a permanent resident, and you’re allowed to live in the US permanently. Irrespective of employment.