I've never had anyone ask me that, but that's because every sort of healthcare provider I have, needs to know I have CF. I don't see why you wouldn't tell them to begin with since they always ask if you have other health problems, or what medications you are on. I just don't see any way around it. Even my eye doctor knows, since I have CFRD he makes sure my eyes are doing ok and such. (though now that I think about it, I'm not even sure cfrd produces the same eye problems that regular diabetes does...)
Anyway, yeah, I suppose, if they are being rude, I'd just say 'Because I qualify for it' but in most cases they should know anyway. Now, if they know you have CF already and ask, they're just dumb. <img src="i/expressions/face-icon-small-wink.gif" border="0">