Nate might file a grievance

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.