As a general rule, if a player hasn't played long enough to have cemented a permanent legacy, he also hasn't played long enough to act like this is his last best shot to win.
When a guy signs a contract and is asked, "Is this your last contract?", then it's probably safe to say that no one can fault him for wanting to go out on top. No sane person could criticize Gary Payton, Karl Malone, Ray Bourque, or anyone who played a long time, came up short in their prime, and wanted to take one last kick at it even if it meant leaving. No sane person could question the commitment of Craig Biggio or Jeff Bagwell, who valued loyalty more than being a one- or two-year mercenary.
But yeah, plenty of reasonable people are right to question a situation like this, where a 25-year-old who's seven years into an NBA career acts like he's in the worst possible situation, even after a 127-37 record the last two years. This would be like Peyton Manning leaving the Colts after his 49-touchdown 2004 season and going to New England, and an NFL career is a lot shorter than an NBA one. You don't think he'd be absolutely vilified for doing that, and rightly so?
There's two things about this whole situation that bother me.
1) Chris Bosh. "I'll go with you, Dwyane and LeBron, just as long as it's not Cleveland." Apparently Bosh, who is regarded as a philanthropist, recognizes that Cleveland is a city that's bursting at the seams with money and doesn't have entire neighborhoods that need bulldozed (Glenville, for one).
2) LeBron acting like he'd be stuck in Cleveland no matter what. He suggested that he doesn't want to be 31, on bad knees, and title-less. Of course, it makes the assumption that, if he were clearly at the end of his career and the team was further from contention than ever before, that the Cavaliers would sit on their hands and do nothing? If he were told "Your knees are in bad shape, and you maybe have a year or two left", don't think for a second that the team wouldn't move him for that last shot on a contending team. To suggest that they would do anything else is insane, because it's easier to risk the wrath for not putting together a contender in 13 years than to keep a local legend locked away out of spite or callousness.