From the Globe, via Slam
Robinson’s agent, Aaron Goodwin, facilitated the deal with the Celtics in hopes of getting his client that additional $1 million. Now his representatives are attempting to determine whether the $1 million the Celtics saved prevented them from exceeding the NBA luxury tax, which would cost the team $2 million for surpassing $71 million in player salaries.
Robinson spent the final two weeks of the season in coach Doc Rivers’s doghouse and he and Marquis Daniels were taken out of the rotation in early April. If the Celtics’ salary cap at the end of the season is close to approaching the luxury tax, Robinson’s representatives could file a grievance with the league that could potentially force the Celtics to pay Robinson the bonus.
I'm going to take a stab at what happened here:
The Celtics got Nate for the obvious intention of helping the team. When it started to become clear that he wasn't going to be in the playoff rotation, the C's figured they might as well sit him and forgo paying him the extra $1 mil (which would be $2 because of the luxury tax).
That's a total guess... but I wouldn't put it past any team to try and save itself $2 mil rather than pay it out to a guy who'd be getting garbage minutes.