There's a number of different thing sthat could happen with Pierce this off-season.
-We keep him at his full $15m (approx)
-We release him, and owe him $5m
-We trade him
-We amnesty him (owe him the full $15m, but it doesn't against the cap)
-Pierce retires (which could entail a number of different scenarios of it's own)
It's really just too hard to tell which direction Ainge is going to go in, with there being so many options.