T-Mobile and such give manufacturers in advance all that stuff. If you ever looked at all the "leaked" software is always from manufacturers and always with the carrier bloat? If carriers added it themselves prior to delivery it wouldn't have that bloat if the manufacturer leaked said software.
The only thing carriers do is beta test after the manufacturer finalizes their code. If Carriers find a problem or don't pass the update it goes back the manufacturer.
There's more than a handful of write ups people have been ignoring for the past few years. These are all written by Google, Sony, Motorola, and recently HTC. Carriers are a lot less involved than perceived by the general public and media.
No one noticed Google backed off on carriers about updates? They worked on the update process that affects certification (WiFi, BT, and such) with governmental bodies. Why do that? Well because at times the longer you take to submit the longer this can take(this has been proven it can take up to 1 year alone at times to get past this step). This stuff happens long before carriers are even involved.