You viewpoint don't seem to be very convincing if snark is your best response to that question.
It was a very "have you stopped beating your wife?" question. Snark is not an inappropriate response.
It is inappropriate if you want to convince people of your viewpoint, which is what I stated. The aptness with respect to other goals could vary of course.
This is a casual conversation, not a trial. Refusing to treat inappropriate questions seriously discourages more questions like that in the future, which is also a solid goal.
In that case I'm sure you could think of better way to discourage nonproductive dialogue - rather than being snarky - if that is the stated goal. At least something less self-defeating.
Either way I think the question was warranted. Especially since the long passage from the Unabomber's manifesto they chose to quote when pressed for a response was rather illuminating regarding their beliefs.