I'm predominantly a scratch baker - due mainly to the fact that I love to bake, and so really enjoy the process of all that measuring, sifting, mixing, folding, etc. It relaxes me, so I do it
And, I admit that I generally prefer both the texture and taste of a good old fashioned scratch cake - interestingly enough, that is what I grew up with! I do believe it's all about what you're used to - so many people who grew up on straight mix cakes (ie: no doctoring) find the texture of full scratch cakes to be heavier, drier.
I think people are asking you because,
a) people are just nosey sometimes
b) they're confused! Your cake doesn't taste like the cake they make from the box (with all those added ingredients, why would it?)....yet it's lighter and has a softer crumb than most scratch cakes they've tasted. How do you DO it???
(Hence, my opinion that they might be subversively digging for the recipe, as Wendy suggests!)
Also - if you're selling cakes, I think people sometimes ask because they believe that your prices should be less if you're using a mix. Never mind all those eggs you add, or the butter and sour cream you doctor it with, the costs of your equipment, or powering an oven for an hour, or those endless hours spent decorating it beautifully.........Betty Crocker was on sale this week for .69 a box, so you should be selling that cake for lots less than the bakery down the road!
I wouldn't feel guilty at all - and I wouldn't hesitate to call the doctored cake you're making "scratch", at least as far as a customer is concerned - customers don't appreciate the difference between a straight up cake mix, and a doctored one, and I don't think that's fair on you. You're adding a heck of a lot of stuff to some pre-measured dry ingredients, and that makes it a heck of a lot more than a standard mix, IMO.
The point about "when is scratch truly scratch" is a good one, too - the fact is that we all use convenience products where we seek convenience. If I use pre-made tart shells for my (from scratch) frangipane and cherry tartlets, can I really consider my tarts as made from scratch? To that end, I've never even been in a bakery that didn't sell tartlets in those well-known little foil cups, with their perfectly crimped little edges! Know what, though? Many of those tarts have been darn tasty!