I think you overestimate the intelligence or willingness of footballers to read or comprehend the contracts they sign.
They'd have a team of lawyers and advisors around them telling them what they can and cannot do.
If their agent tells them to sign the contract, they'll sign it.
Look back to when Robinho thought he was signing for United, they're generally either A. This as shit or B. Couldn't give a shit, neither A nor B should give you the confidence of a footballer knowing the finer details of a footballing contract.
On a separate point, Foster playing for Watford is an entirely different contract to Ronaldo playing for Manchester United, no offence to Watford or Foster.