In one word: LAWYERS.
Seriously though, I worked twice in my life as an independent contractor, and both times I signed an employment contract.
There are lots of stipulations the employer can and does include; maybe some are not entirely appropriate/within their rights, but it would cost the employee money to find out and fight it if the need arose.
Whether the terms and conditions are within legal parameters or not, once you sign the dotted line, you are bound by everything.
Contesting anything is a major hastle.