Publishing remarks made as background when you make it clear that they were made as background and that they weren't part of an actual interview is not unethical--though it is still unfair to the person being quoted, IMO, for reasons which are well explained by pg in his article.

Publishing remarks made as part of background for a completely different topic, in such a way as to make it seem as though they were made during an actual interview on the topic quoted, is unethical, IMO. And since that's what happened, I stand by my remark.

Publishing anything that's not presented in a "this is blatantly on the record" or "this is blatantly in public and observable/reportable by anyone" context is unethical. Unfortunately, this happens all the time because it generates a lot of buzz for the outlet, just as happened here. In fact, there are outlets whose entire business is made up of publishing "off the record" stuff, like tabloids.