"You broke your promise that I'd have unlimited publication rights" is a sad story of broken promises but it's not a story of journalistic decline.
Sure, but that isn't the central issue here that Greenwald is claiming.
Maybe this is breaching the agree Greenwald had with the editor(s). But independent op-eds able to say whatever they want isn't a principle of journalistic integrity. Questions of that sort revolve around whether you lie or misdirect or run paid articles and so-forth. That you refuse to run an op-ed right when your writer really want it published is a wholly different question, one clearly about the paper having indeed evolved to a left-liberal position while Greenwald maintains his conservative/Libertarian principles - ie, the argument is political.
That's not forbidding him from publishing it, it's expressing regret that he appears to be unwilling to address the editor's comments on his draft.