He was not forced to do anything - he was given a choice: either we continue to pay you, and you abridge one line that we don't like being associated with our product, or you keep on saying whatever you wish and we have no requirement to pay you.
Both parties are free to set any terms (as allowed by law) on the deal which both will agree to. If both don't agree to the terms, you have a standard failure to negotiate a deal. If one does something in the future which causes distress to the other party, the other party can certainly demand redress. If the party which caused the issue refuses this, why should they be forced to submit to his whims with no freedom of their own? And what does that have to do with freedom of speech? He's still saying what he wanted to say, is he not?