It's kind of a fine line in a situation like that... where you've been allowed access somewhere that the public isn't, but you're in there as a representative of the media, whose job it is to report back to the public. On the other hand, since Haynes' future stories are going to depend a lot on relationships with players, he has to be careful. Alienating Shumpert (and potentially LeBron James, whom Shumpert was talking to) is a big risk for what seems like a relatively small story.
I've heard stories of reporters hearing something said between attorneys in an elevator, putting that in a story, and the reporter suddenly finds all access to future courtroom stories evaporating due to the attorney's reaction. Who knows - the reporter may actually be within the bounds of propriety (I don't know what the media's rules are, once they're allowed inside the Cavs' locker room), but given the reaction here, it either wasn't clear to the players, or to Haynes - or it needs clarifying. In the absence of clarity (maybe), Haynes ran with the story, and didn't do any relationship-protection with the players involved (at least not with Shumpert - maybe he told LeBron, but I doubt it).
Maybe Haynes thought this story was important enough to risk the downside. I wonder if he still thinks that.