Supporting the upstream is different. One cannot simply download AOSP and install it on your phone, however custom roms provide something you can actually download and install. Google deliberately does not improve the AOSP base applications because they don't want people using their proprietary apps. Google mainly supports AOSP because they can get more people hooked on google services with it than they loose from things like Kindle.
So directly supporting an easy, installable downstream with bells-and-whistles like cyanogenmod would be a serious threat to google.
So directly supporting an easy, installable downstream with bells-and-whistles like cyanogenmod would be a serious threat to google.