It's not. As https://developers.facebook.com/docs/chat/ explains, "Facebook Chat should be compatible with every XMPP client, but is not a full XMPP server. It should be thought of as a proxy into the world of Facebook Chat on www.facebook.com.
That's not to say I wouldn't like to see them improve their support for all of XMPP though.
Really, I can only think of laziness and trying to build a walled garden as reasons to support c2s but not s2s connections.
Both of which are not exactly good explanations for one of the largest companies in software engineering.