I'm sorry, but I had to do it. A rhetorical greeting is a greeting phrased in the form of a question, that is by definition rhetorical if and only if the greeter and greetee consciously but covertly agree to treat it as such. Consider this basic example: person A approaches person B and asks "how's it going?" At this point, person B can say "fine," and render the greeting non-rhetorical, having answered the question. On the other hand, they may choose to either respond with another greeting (perhaps rhetorical) like "how's it going?," or they may decide to ignore person A entirely.
If the greeter receives the same or similar question rather than an answer to said question, they have the choice of forcing the greeting back on to the greetee, and thus can convert a rhetorical greeting into a non-rhetorical greeting that absolutely must be answered in order for one to not make the wrong impression. For example:
Person A: How's it going?
Person B: What's up?
Person A: Not much, how about yourself? (conversion occurs here)
Person A: Same shit, different day.
It is debatable whether it would be considered "rude" to treat all greetings as rhetorical, or whether it would be considered "rude" to force a rhetorical-greeter into actually answering the question posed. This all depends on the circumstances. Personally, when I am greeted in such a manner, I answer the question, and then offer my own greeting. If I am the greeter, I find it rather obnoxious to force someone by converting my greeting into a non-rhetorical one. If you greet someone in this manner and they respond with the exact same question, chances are good they may have other things on their mind, or they possibly just don't like you, or in certain cases, they are not as familiar with the English language and have yet to develop an internal array of greetings, rhetorical or otherwise.