Why did Google block Facebook Contact Sync on the Nexus 5? 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:


  Since Facebook contacts cannot be exported from the device, the
  appearance of integration created a false sense of data portability.
  Facebook contact data will continue to appear within the Facebook app.
  Like all developers on Android, Facebook is free to use the Android
  contacts API to truly integrate contacts on the device, which would
  allow users to have more control over their data. We are removing the
  special-case handling of Facebook contacts on Nexus S and future lead
  devices.


TechCrunch's coverage on this:
http://techcrunch.com/2011/02/22/google-android-facebook-contacts/

Engadget's coverage:
http://www.engadget.com/2011/02/22/google-disables-contact-sync-in-facebook-for-android-only-nexus/

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.