If I had a choice to sign the agreement or not, of my own free will and with no incentive, I may actually do so. Like you, I have no particular desire to speak ill of anyone, deservedly or not. Time spent disparaging a former employer is time which could have been better spent moving forward.
But when you offer me an incentive for signing the contract? It makes me wonder why these rights I'm giving up are worth so much to the company. And feeling like I'm being bribed or pressured to do something is one of the best ways to manipulate me into not doing it, just because I'm so stubborn :)