They aren't going to take on a contract like Hedo's. They want to gun for another title run next season, but aren't going to kill the potential rebuilding process along the way. Dude's got four years left on his contract ($12M per the last two seasons); Celtics are going to be very reluctant to take on any salary obligations that extend beyond KG's deal, much less THAT much money for a guy that's virtually guaranteed to be in serious decline by then.
Given that Rondo will still be under contract, as will likely be Pierce and/or Perk, add Hedo's contract to the other inevitable players we accumulate and there's no prayer we're under the cap for four seasons. Danny's not doing that, madate is to win but not at the cost of not being able to really rebuild for that long.