The reason for this restriction is because most US airlines sell access to lounge through payable membership programmes rather than give it to their elite members, as is normally the case in Europe.

You get comparable restrictions in other alliances although these are limited to the US airline FFP members (eg: case of UA for access to RCC in *A or North American members of NW WP)

If elites were to get access to lounges when flying domestically in the US, why on earth would they shell out for a distinct lounge access programme?

Which leads me to my question: is the international flight only rule actually enforced at CDG and MXP/FCO, as least as regards other European Skyteam FFPs?