While i also have never been asked, I think I would go with something along the lines of "I'm not funding terrorism, and although some might not know its fake, I would."
And then of course I'd likely go off on some rant about how they are classics that are worth saving for and if people can't afford it, then they shouldn't wear one... and that its none of their business!
(I personally just don't understand buying fakes, especially when we all know that there are some are lovely brands such as coach, dooney and burke or tano that aren't as expensive but are nice as well!)