Re: why no celtics on the USA team?
Very simple explanations:
When Jerry Colangelo began the US National Team process in 2006, he wanted to form a core group of players that were 100% committed to practicing and competing in the 06 Worlds/07 Qualifiers/08 Olympics.
Allen and Garnett both declined invitations. Both of them played on the 2000 Olympic team, and were not willing to committ three more summers. Pierce was named one of the 24+ original core players and accepted, though his position was solely a PR move. Basically, the public attitude was that Pierce was a negative aspect of that disastrous 2002 World Championship team that finished 6th. Jerry Colangelo decided to add him to the core group (in name only) both as a public statement showing that USA Basketball wanted guys who played hard like Pierce, and as a personal redemption for Paul himself.
Kendrick Perkins and Rajon Rondo were both invited to the USA Select team comprised of up-and-coming NBA players in order to scrimmage against the Senior National Team.