I purchased a Nexus One shortly after launch, and it did not come with Twitter installed. In fact, the Twitter app didn't exist then.
I _did_ however do a hard reset the other day (ugh...) and all of my stuff was gone. Twitter, Amazon, and Angry Birds Holiday seemed to still be there...
But looking at my phone now, a bunch of other apps that I had downloaded are back, mysteriously. I dunno.
Those are not carrier specific apps.
The reason why they are there because there were no installable apps at that time. I do not need to sign in to any services or accounts to use those apps.
Amazon MP3, Facebook apps are tied to some services that requires user accounts.
One pre-installed app on iOS that fit the category is YouTube App.
Specifically: Amazon MP3, Twitter, and Facebook.