It's just a case of bad circumstances I think. I definitely wanted to keep Powe, on the exact contract he ended up getting. I fully believe Ainge (even though I often don't trust him) when he said that he offered Powe this contract. It sounds to me like the problem was the timing of things and Powe's feelings.
As to timing, Powe went into the offseason thinking at worst he would be able to get the qualifying offer, make a little over a million rehabbing then become an unrestricted free agent next year, when he could likely net $3 or $4 million a year from the Celtics or somebody else. It was logical for the Celtics not to offer that deal because they understood the market when Powe didn't. They were willing to pay him for rehab, but wanted the option of one good healthy year from Powe (2010-11) at cheap money if they did so. So would (and did) any other team interested in Powe. But since the Celtics were the first to say this, and Powe was just not prepared for that reality, he took it as an insult. A month and a half later, he accepted the same offer. Timing.
Second is Powe's feelings, and I'm not going to get into whether he was actually disrespected by the Celtics. By feelings I mean it's almost like Powe wanted to get a hometown bonus instead of give a hometown discount. He expected the Celtics to reward him for helping them win a title. They looked at it realistically. As much as I think Powe deserved a reward for all he's done for this team, they couldn't just offer him a one-year deal and not get some option at a healthy season. It just doesn't make sense. But he wanted that from Boston - the one year deal. (This doesn't even get into his complete misinterpretation; as has been pointed out, Boston didn't make a QO to Tony Allen last year and ended up giving him 2 years, $5 million. Not giving a QO to Powe didn't mean the Celtics weren't interested.)
I felt strongly when Powe made his first murmurs after the failure to make a QO that Ainge was wrong to be alienating Powe after the model Celtic Powe was. Frankly, now, I think Powe himself is responsible for a lot of the alienation. And that's not to insult Powe, a guy I respect and will continue to follow and cheer for, that's just to say it looks to me like Powe wasn't really insulted, he was just oversensitive and caught in a situation he wasn't prepared for and didn't handle with logic, but with passion. Can't blame him, he seems to be a pretty sensitive guy ruled more by emotion than reason.