While the new CBA, thankfully, closed the loophole that allowed players to be traded, waived, and re-signed by their old team, I assume it still permits players to be waived then re-signed by their old team.
End of the bench guys are often waived and re-signed during a season, depending on a team's needs for depth. One example of a non-minimum guy who was waived and re-signed is James Jones. To create additional cap space of almost $3m in the summer of 2010, the Miami Heat waived Jones, who had signed a five year, $23.2m contract. The final three years, beginning with 2010-2011, were partially guaranteed for around $2m each. A few weeks later, after the Heat had signed LeBron James, Chris Bosh, Mike Miller, and Udonis Haslem, Jones re-signed with the Heat for the minimum salary.
Paul Pierce is scheduled to make $15,333,334 in 2013-2014, of which $4m is guaranteed ($5m if he he reaches certain playing time marks) according to ShamSports. That year becomes fully guaranteed if Pierce is not waived on or before June 30, 2013.
Can you see a still-capable Paul Pierce agreeing to be waived and then signed for a smaller contract (the minimum, even) to create cap space to sign a free agent (Paul Milsap will be a free agent in 2013 for those who want a classic power forward who plays in the low-post) or trade for a star?
It's an option that allows Pierce to remain a Celtic for life without paying him like an All-Star if he declines to below an All-Star level. Perhaps the contract was even structured so that this was the plan all along to have this as an option for cap flexibility.