They don't, I never said they did. However, to get a return on their investment they need a successful, profitable business. Pissing of the employees isn't going to achieve that.
Employers do offer T&Cs and then if you agree to them you sign a contract to take up employment. That doesn't mean either party can arbitrarily change the terms of the contract after that. Changes can be made but both parties need to sign off on them. It's a lot easier for a company with out...