I think it's hard to tell somebody that he has to spend his entire career in a city simply because that's the team that drafted him.
Nash is comfortable in his living and playing situation. Carmelo isn't, for whatever reason. Since the money will be the same, or more, if he stays in Denver, it seems like he has priorities outside of money. I can't hate on a guy for leaving to pursue happiness elsewhere.
The only time I get upset by FAs moving on is when you have a situation like Lebron's, where the organization catered to his every wish and he still couldn't give them the respect of being straight up about his intentions. That was handled in a no-class manner, and Lebron deserves the ridicule he got.
However, Carmelo isn't doing that. Rather, he's told his franchise he's leaving, and is letting them shop him for compensation. He isn't threatening to block trades, he's just not committing to an extension right now.
Is the Carmelo situation so different than the KG (or Pierce) situations, where KG talked about opting out and leaving, and Pierce talked about "pulling a Kobe" and demanding a trade?
All that said, I do like Steve Nash. I'm disappointed that the organization hasn't built a winner around him.