Because they're worried about his effect on the team. The suggested reasons for that vary....some think it comes from KG (based on their history in MN), some have related it to Rondo, House, and Pruitt losing playing time, and others feel that Doc Rivers doesn't want Marbury at all.
By signing him and letting him go after March 1st, they would keep him off the playoff roster of the Cavs, the Lakers, the Magic, and any other team that the Celtics would have to worry about in the playoffs.
wicked false...
NBA playoff rosters are set April 15th, a team can sign any free agent player between now and that time and have that player as part of their 13 man playoff roster...The Mach 1st deadline refers to bought out layers on. A player must reach a buyout agreement with their team by March 1st in order to be eligible to play for a team in the playoffs, they can of course play for the team the remainder of the season if they like...In effect, now that Marbury has signed with another team, the Celtics and he has officially met the March 1st buyout deadline, if he is waived by the C's, he can in fact sign with any team and be eligible to play in the playoffs...
