I really don’t know. I’ve never had any experience with them outside of temp services and the last thing they wanted was a union.
I was suggesting that they appear to work in a similar manner and provide similar protections to the employer - independent of any law. If the workers get uppity, pull the contracts for a safe reason and bring in replacements more willing to work.
The largest reason why those agencies don’t want unions is that they would scare away contracts.