Because the teams needs to show that throwing a tantrum will not be rewarding to their players. If he goes somewhere else and signs for big money after not honoring his contract then he wins. The team needs to send the message that a contract is LEGALLY BINDING, and its entire purpose is to ensure that regardless of future situations, THIS IS GOING TO BE WHAT HAPPENS, not this is going to be what happens if you feel like it.
Contracts are becoming totally pointless and jokes in the NFL as a player can not honor them if they don't feel like it, with relatively little consequences. If a team cuts a player he has to recieve the remainder pre-determined guaranteed money. What if a team cut a player and said "we think you were overpaid based on your performance, so we don't feel like payinig you the rest of this moeny that we both previously agreed we would." I don't understand why the franchise must honor the contract, at least in terms of payment of guaranteed money, but players don't have to honor them in terms of guaranteed years.