Part of the terms of service you agreed to when you registered the device said that BN can modify the stock software of the device. So actually you did authorize it.
If the whole world actually read TOU's everything would run so much smoother...
Yeah, right, this is just a simple matter of the terms of service.
Are you a B&N employee?
Some people's minds are like fruitcake, stale, half baked and seeded with ingredients made elsewhere.
I'll come out and say this, I do work in B&N retail.
The Twitter app was pushed out with Nook Video, though I had thought it was only to the Nook Tablets, not Nook Colors. Both got Facebook. It's possible I am mistaken about the Twitter app for the NC's.
I know there is an issue with it on the NTs which is under review and hopefully will be resolved soon.
Despite his slightly brash way of putting it, FT is correct. The terms you agreed to when you created your Nook allow for B&N to add apps and make modifications. Which was why you got Nook Video recently, and various other apps for free over the years.
As far as your optiions, I don't think you'll get far returning the NC. It's possible if you ask nicely, calmly, and politely that exceptions will be made, but you're well outside of the return policy for the Nook. You may also have better luck returning it through the customer support number you called as they have more range than the stores do when it comes to Nook return/exchange issues.