Yep, and that's fair enough. The psychology is interesting though, in that if you didn't know it was on the disk and didn't know it was produced at the same time then would it be any more palatable?
The DLC (and even expansion packs of old actually) are often done during the main production time, basically because it's cheaper while everyone is already being paid/tooled up. It also used to be a good way to extend development team time while the game enters QA, certification and the mastering i.e. keep them around 'on title' before they switch teams, especially if issues are found and art/sound/coding production schedules don't line up.
I agree it does seem bad to 'trim off' good meat just to sell later, but then that's a risk few publishers can get away with, i.e. Mass Effect 3 was highly desired and sold well, so they felt braver/eviller
on how much they could cut out and sell separately.
If the music industry could do it, they would and probably the nearest analogy would be making money through concerts and just using the album to shift tickets. The 'interactive entertainment' of games just makes it more possible to slice in different ways.
The ethical part is really on the balance of operating a business for maximum profitability versus hitting your sales by acting too much a douche. A balance a lot of publishers struggle with..