These 2, along with KLM Asia, Swissair Asia, Lufthansa/Condor was setup due to political problems.
In order to fly to Taiwan, these carriers have to establish a new "company/franchise" and no flags painted on the plane.
Currently, China is still trying to intervene some air service agreement for Taiwan, but this is rare.
Except KLM Asia, all European carriers' "Asia" franchise has been dropped due to poor revenue.
JAA's case is different, right now it has been designated as an airline that specializing Taiwan route. So JAA's new livery will include Japanese flag, which they weren't allowed to do so in the early days.
NW serves Taiwan back in 1947, and UA bought Pan Am's flight to Taiwan in 1985, so there were not much political issues involved.