Basketball is a sport. The NBA is a business.
Doc Rivers coached his team throughout the season and playoffs. He didn't refuse to show up or coach the team throughout all of this so he didn't quit on them. After the season is completely over, it's fair game to look at other coaching and playing opportunities for everyone. The NBA is not some rec league where a coach/player can just up and leave his current team during the season to coach or play for a better one.
People are saying that he quit on the Celtics is just flat out hilarious. The contract stipulations that a coach signs is different from a player's contract. The media knows that most people aren't really familiar with some of the business side of sports, so they try to exploit players and coaches when they make business decisions that will not be widely accepted.
The media loves to stir up controversy. It's part of their job!
Really?

You do realize there was a no compete clause in Doc's contract?