The different systems is most a historical issue and what systems were available for a a particular aircraft at a particular time. And likely getting lower price system for the domestic market versus using the WW vendor like Panasonic. Believe the DirecTV equipped aircraft were limited to Thales and not available on non-DirecTV aircraft.
.... It counteracts the usefulness of buying subscription packages if you can't use them across the fleet.
That should not (but I don't have the personal experience to valid UA's statement) be an issue {when the wifi works
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