Three words. No Trade Clause.
Kevin Garnett, Ray Allen and Paul Pierce are at that stage of their life where they don't need more money. Of course, they will not play for free. Being able to retire with dignity playing for the team they want is more of a priority. These players do not deserve to bounce around different teams every season for the rest of their careers.
If Danny Ainge presents them two options, one with more cash but less security and another with less cash but with a no-trade clause, they would lean towards the latter. The Celtics will have more money to play with during the off season and trade window. Right now, they are experiencing a renaissance of sorts. When the numbers do go down, that is still one hell of a bench.