Boston fans are very knowledgable. They are respectful of good teams and players. They know their history and are probably smarter than any other fan base in basketball. They were simply showing respect for one of the greatest basketball players of all time.
That was no different that when Boston fans shouted "Beat LA" following the 1982 NBA Eastern Conference Finals when Dr. J and the 76ers beat the Celtics in a game 7 at Boston.The Celtics were the defending NBA champions. And the game would go down as Bird's only game 7 defeat in his entire career.
During that time the 76ers were the Celtics biggest rival. There even use to be a saying in Boston, that Nationally its Celtics/Lakers but locally its Celtics/76ers. When I was a little kid, I hated Andrew Toney and Moses Malone!
Here, this chant is actually considerable louder. It is remembered as a classic moment in Celtics history.
Its called class (and a lot of Laker fans in the building).
Nothing to explain.