You never know. About 3 months ago I entered security code to N950 for Track & Protect. Of course by the time I flashed the new FW some weeks ago, I had forgotten all about it. Upon activating Track & Protect again, the phone was asking me the security code once I booted. I had a hell of a time remembering what it was .

So, it could be that the branding is also put into a protected space where flashing doesn't erase it. Doing it for the security code makes sense. Some carriers might feel it makes sense for their branding too