An employer can't make you sign a contract that says "...and I will be your slave forever and will never work for another company."
"I'll be your slave forever", no, they can't. "I will never seek any form of employment anywhere else", yes, one most certainly can enter into such a contract. Even if no future considerations are given. It may not be prudent, but one can enter into such a contract.
As long one enters into it with full knowledge (not likely), one can agree to such terms. People exchange immediate considerations for future opportunities all the time (selling proceeds from future rights for immediate cash payout would be one such example). And no one "forced" anyone to sign a contract. Employment can be employment at will without any contract restricting the terms beyond those established by laws.
Having said all of this, I still don't understand how they can sue Apple for violating terms of a contract to which Apple is not a party.