Along the same lines as the conspiracy theory about absence of Google Talk on G1 -
Fact #1: mobile carriers charge customers for airtime.
Fact #2: UMA allows customers to talk using a WiFi connection (sometimes a flat fee is involved).
Fact #3: this strips mobile carriers from airtime revenue that could otherwise be produced.
Speculation #1: Carriers offering unlimited UMA calling on a flat fee basis realized, to their dismay, that a lot of people use it a lot.
Corollary: UMA will be a feature that is offered less and less in the future, until it disappears completely.
G1 may have been the harbinger.