The missing rationale here seems to be:

* The domain part of a URL is a critical security indicator.

* The shorter and simpler this indicator is, the more chance users will understand and trust it.

* Very few users trust www.yahoo.com while not trusting yahoo.com.

* If we assume that users who trust www.domain.com also trust domain.com and vice versa, we can remove the "www." from the UI, creating a shorter, simpler and more understandable security token.