There's a huge difference between the two situations. Until this summer, the Cs were trying to contend, and loaded up their roster with mid-salaried veterans on multi-year deals to play with KG and Pierce. Since their title, Dallas has been in salary clearance mode, hoping to snag a top FA this year and next to build another championship team around a discounted Dirk in his final years.
Dallas also has a reputation as a coveted FA destination, while Boston doesn't. Boston moved on from their franchise guy's final years because they couldn't reload enough to be a contender again. Dallas' plan is to reload this year and next around a discounted Dirk.