I agree wholeheartedly that Tweeting something like "I am going to rape you" is vile, despicable, abusive and "threatening" behaviour, and that the poster should be prosecuted for such, but unless it is intended to be, and can be, carried out, it is not a "threat" in the real sense.
The media and police, in this case, are stretching the definition for nothing more than effect. If the man in Manchester can prove that he had no intent or ability to carry out what he Tweeted any good lawyer will get him off. However, had he simply been charged with "xyz behaviour" he would be bang to rights.
Perhaps Di is on the right track, maybe the legislation needs to catch up with the technology.