Being that they opted not to extend his contract. I think the Rockets still want to keep Griffin and is a part of the future, but they just want to evaluate him on a day to day basis. The Rockets don't want to give the impression that just because a guy has talent he can do what ever he wants.
They do retain his bird rights, but do not retain the right to match (which they would have if they had offered him a 4th year).
I think the ROckets did the right thing by not re-signing him, because once EG comes back, he'll be forced to play his very best game to increase his free-agent value and make all the other teams overlook what happened this past pre-season. Plus, the ROckets could always re-sign him, though EG, understandably, might not take up another contract. Either way, EG is going to have to play his best game ever and contribute to the Rockets this season if he wants to secure a decent future in the NBA with other teams. He has a lot to prove now, and a lot of critics to silence. When EG comes back, and I expect him to come back and play at some point, if he cares about continuing to play in this league and getting paid what he's worth, he'll bring his A-game. If he comes back and mopes around on the court, teams will only be willing to pay a pittance for him. He's past the point of being taken on potential alone.
