Android OS is open, all Google products are not open. That's how Google stays on top. This was a big contention when the Android OS CyanogenMod ROM was getting shipped around with Google Apps in the ROM (Gmail, Market, Maps, etc...) and Google sent their lawyers after the developer.

Android OS is just the OS framework, not the sync services nor the applications that make use of the sync services.

I remember Ari wrote a blog post about services awhile back, guess Google beat Nokia to the punch to indispensable services.