I like it for two reasons, one Stevens has to get players and other GM's to warm up to Boston after DA left some bad tastes in their mouths.
So showing other players that Boston is loyal to their players is a good move.
Also I could see if they win the Finals that Al might want to go back to Atlanta for his farewell tour, finish it up where it started. It would be a bad move to send him to a bad team for extra talent, so Atlanta would make sense. A team with some serviceable young players that is also in the hunt who could use Al's leadership and mentoring.
But I'd rather him stay and retire a Celtic, maybe be a coach too. He seems to have everyone's respect, it's really worth having him here.