The 3ds game will come with ONE installation code in the commercial packaging next to the manual. You connect to nintendo's network, enter the code, register the ONE install to your ds, then install it from the slot.
The code is used and registered to your ds, you can no longer install that specific copy unless you get another code from buying the retail game. At best you can install the game to yours and let a friend use the cartridge.
If you need to transfer the code to a new 3ds xl or whatever, you go back to your first ds and go to nintendo's site, they will uninstall the game from your current ds and you may use the code for a new ds, but only 1 time per model. Anything else and you need your original cartridge. You should have those anyway unless your sharing games left and right, so no need to complain.
Problem solved.
The code is used and registered to your ds, you can no longer install that specific copy unless you get another code from buying the retail game. At best you can install the game to yours and let a friend use the cartridge.
If you need to transfer the code to a new 3ds xl or whatever, you go back to your first ds and go to nintendo's site, they will uninstall the game from your current ds and you may use the code for a new ds, but only 1 time per model. Anything else and you need your original cartridge. You should have those anyway unless your sharing games left and right, so no need to complain.
Problem solved.