I'm still not seeing where the problem is. The Bruins initially offered two years, Brad wanted three, they allegedly eventually agreed to three (which personally I never would have but whatever) but only for a certain price. We don't know what that price was. But they'd already compromised on the term; clearly for whatever reason they were not willing to go any further on the money. Impossible to judge whether that was justified or not without knowing the $$$ on the table.
I take your general point. Sweeney has a history of going quiet and just shutting up shop on guys he's decided he doesn't want around anymore - there was the same sort of smoke around the departures of Krug, Chara and DeBrusk, and probably a few others besides, and of course the Sway business got very messy. There's a ruthless, perhaps bordering on unpleasant streak there, and on top of that Don definitely isn't always entirely truthful with the media. I'm just not sure how much that applies here. There were talks, both sides gave some ground, eventually management reached a point where they'd give no more, and having reached an impasse with Brad they traded him.
That's playing hard and cold, for sure, but I'm not seeing it as outright awful. Again, really depends on exactly what Brad wanted. I'd have expected the Bruins to be reasonably generous, but there's a limit. At some point you do have to say "this much and no more", no matter who it is.