it's not just the carriers. It's the manufacturers too - I know of no phone (aside of the Google developer phones) that would allow you to freely install your own build of the OS or even just remove "value added" software that has been installed for "your" "convenience".

Unless you exploit security holes in the vendors crappy security systems. The fact that they don't even invest enough resources into a quality security framework (which helps increasing their revenue) speaks volumes of the quality of the other "improvements" they make to stock android.