Convinced of what? That that agreement HAS been struck? You shouldn't be, since we have no reason to believe it has. But if you're not convinced it could happen and that it would be a pretty sound (or at least net-neutral) financial decision, I don't think you're looking at it from his perspective. Bonner is one of the best shooters in the NBA. He will get another contract, and probably a pretty substantial one ($10M/3 easily). Even a minimum deal is $1.4 Million for someone with his experience. If the team guaranteed him $600k more, then he'd make $3 Million. Also, if he gets claimed on amnestied waivers, he'd get all $4 Million guaranteed.
It's 11 days in real life, but it's actually just a blink of an eye for the off-season. We're talking about the Spurs amnestying Bonner before the first free agent inks a contract. There will be plenty of money left over.
So yeah, no one's saying that Bonner has agreed to this. But we are saying that he would really not be taking a gamble by agreeing to the extension. I don't know if anyone has agreed to this very thing before, since the amnesty waivers are such a new thing. But players tweak their contracts to help their teams out all the time.