The difference is that I am incredibly confident that KG would return to Boston if Pierce was kept, same for Doc.
KG is huge on loyalty, but in this case his loyalty lies with his close friends, Doc and PP. I'm pretty sure on the inside, KG and Doc have pushed to keep the trio together, but I don't think DA at such a pivot time for the team, had much interest in bringing a declining PP back for $16M...and I in a way I don't blame him.
But KG, Pierce are like brothers, and would take any opportunity to keep playing by his side. DDoc loves both those guys and would love nothing more than to coff h both again. Right now, the Clippers are probably the only team interested in taking all the (and letting them play one more year together), and hence the desire to move over. I don't think KG could beae looking at pierce as an enemy after all they have been through.
Danny's apparent decision not to bring pierce has pretty much pushed this in motion, by you really can't blame him either because it's his job to do what's best for the team's long term future.
So in this case it's a complicated situation and I understand each parties position. Also Boston looks clearly in rebuild mode now (or at least re tool mode) so it's the last chance for those guys to make a run at a title together. Boston just came off a first playoff exit, two of their most promising players (Sullinger and Rondo) are coming off major surgeries and they'd no idea of how effective they'll be next season, there is no cap space to improve the Tam, and there are a stack of mediocre players on long term contracts. Not promising scenario for a 37 year old future hall of fame and all star who only has a year or two of glory left.
Rays situation was completely different. Boston wanted him back, they offered him a very fair deal, and they entered to keep the entire big 4 together. Ready turned it down straight up and left a team that finished one game short of a finals appearance to go to the very Tam that knocked them out.