Did BB force DA to sign the contracts? Did he handcuff him to his locker so he couldn't go to another team?
Sorry Danny, it takes two to tango.
The cap rules in the NFL plus the fact that most contracts are not guaranteed means that the (figurative) answer to your questions is "yes" and "yes."
Amendola had a contract with the Patriots. And every year, Belichick would inform him that if he didn't take a pay cut to free up space, they would have to cut him. Given how the system works and because BB had total control over the timing of any action, it gave him all the leverage. So, yeah, he had Amendola over a barrel, because DA wasn't enough of a superstar to have all sorts of roster bonuses and/or guarantees in his contract, which allowed BB to say "You can tear up your contract, sign for X and win as part of our team or see if you can hook on to whatever team has an opening when I cut you at the end of training camp."
That's not Belichick's fault, by the way. It's just the system the NFL has in place. But Belichick has been pretty ruthless in how he has used it, and he learned at the feet of the biggest bastard in the game. (Parcells.) So I don't blame Amendola at all for his comments. It's just the nature of the NFL.