Did Obama “warn” Trump against hiring Flynn or did he suggest Trump not do so?  Warning implies a possible danger, problem, or other unpleasant situation.  If Obama knew of such, he had an obligation to tell Trump what it was, but according to the story, Obama told Trump that there were more qualified candidates for the crucial White House post.  That would make Obama’s comments a suggestion and not a warning.   I believe "warned" was used in the story because it sensationalizes the story to best fit the media's anti-Trump agenda.