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.
The reason employers go to staffing agencies/contractors are:
1) To prevent employees from organizing (but this is far from the main reason)
2) To protect themselves from federal & state laws/red-tape concerning full time employees (this is by far the #1 reason, and with the implementation of ObamaCare we’ll see even more companies dumping their full-time staff and going to contract labor)
3) To get just-in-time skill sets (this is particularly true in IT—computers—where technology changes so quickly that companies have a hard time keeping up) which allows a company to bring on contractors for a specific project, then release them at the end of the project.