Is there a particular developer clause that was violated? If so, why is the facility blocked only on the Nexus and not other Android based devices?
I did some digging around and found that this change was introduced in an update for Gingerbread. It applies to all Nexus devices starting from the Nexus S (except the Nexus One since it comes pre-installed with Facebook).
The problem Google has with Facebook sync is the inability to port the data that Facebook sync adds to the contacts, out of your device.
Here is an excerpt from their statement on this issue:
TechCrunch's coverage on this: http://techcrunch.com/2011/02/22/google-android-facebook-contacts/
I am still trying to find the official statement on a Google site. Will update it here if I find it. Also I want to know why Google doesn't get all manufacturers to enforce this rule if it is part of core Android policies.
According to a post in a Google Product Forum:
An alternative to make this sync happen is to use a 3rd-party app like Sync.ME - Sync for Facebook: