Try to take the sports fandom out of it, and inject more of the "business".
Say your boss comes in, and says you're fired, or you're being transferred against your will to a really undesirable location. You tell your family, and then your boss comes back and says "nevermind, we can't do that after all." Four months later, you get a job offer from a rival company, that promises you a better standard of living than you had previously, and which really seems to embrace and respect you and what you can do for the organization.
How many of us say, "Even though my current employer tried to terminate me, I'm still loyal, and would never join a rival company"?
I wouldn't, and I suspect the vast majority of Celtics fans -- or people in general -- wouldn't, either.
To continue to put this into terms people would understand (and ignoring the 'fired' aspect because that isn't at all relevant), you left out the part about money. If your current employer then offered you $100,000 to stay and a clause in your contract that said you couldn't be transferred, but you chose to go to another rival organization for half the money, that would be seen as foolish and your family would probably be pretty upset with you.
But that isn't really the point. I understand Ray's decision; I don't hate him for it; but, I don't think he has any right to tell fans to get over it. He left on bad terms. I agree with Ogaju's response - fans make players. You don't see the world's best lacrosse or field hockey players making any kind-of money. He chose his bed - it was probably best for his career - but now he has to lie in it.