Well, because you don't know what you obtain by a license applicable on most of the countries.
First, you can read about licenses
here. What you've to read and understand, is that owning a copy of a single game, due to the license, doesn't grant you the right to use all copies of that particular game. Or to be said in a more understandable way, if you buy a single cart, you don't have rights over all the carts containing the same game/software.
And how does owning a DS enter in this problem? Well, as you don't own all copies of the same game/software you bought, you have to dump your legally bought game using some sort of hardware. For DS dumps, I know the easiest way is using a DS and a SLOT2 flashcart, iirc. If you don't own a DS, you can't dump your own game, thus you need to download a copy of
ANOTHER (yes, this is the important fact) cartridge, from what you posses no legal rights to use, thus you're using and illegal copy.
And asking Nintendo about emulation is plain stupid, they've a bunch of lies on their web page about emulation that do not relate in any way to the law reality on all the countries I know of, thus being legally void.
So now you've facts based on license law, not like your bunch of assumptions without any base... At least I had a good laugh reading it.