Anyone with a brain knows that Ainge didn't want him there and Doc didn't want to be there anymore.
If this was the case why would Ainge give Doc a five year deal two years ago? Even back then Ainge knew rebuilding was coming sooner or later. Besides, that 7 million doesn't go under the cap, and it's not like the Celtics can't afford it. Coaches like Rivers don't spring up very often. It's in Boston's best interest to keep Rivers around.
BTW, a draft pick that's likely going to be in the 20's is hardly compensation for losing one of the best coaches in the league. They traded him because they refused to let Doc out of his contract.