First off, it's not a bad idea for the normal one too, but because of all the physical goodies, people that buy it on Steam are kinda screwed to buy it twice.
Anyways, SE has already stated before that people who bought the game on Steam need to buy the expansion on Steam. This is fine, except you're forcing people who buy the physical CE of the game and currently own it on Steam to purchase it twice.
By including a Steam code with the CEs, players can get the physical CE, and still be able to play it through Steam when they activate the code.