I do not have definitive answer (opinion) for your last question.
These are regular army units where the enlisted are not native nationals.
The first sentence from linked article about French Foreign Legion says that The French Foreign Legion is a military service branch of the French Army established in 1831, unique because it was created for foreign nationals willing to serve in the French Armed Forces.
I repeat if it did not sink in: Foreign Legion and The Gurkhas are not private mercenary units, they are not private armies.
The main argument of calgoo was that "Private militarizes armies should NEVER be allowed to exist.".
This is very far from being "a very nationalistic position". He did not say that foreign nationals should not be enlisted into national armies.
So please, could we stop this bullshit now?
And what about the European and American nationals who joined the kurds to fight ISIS in Syria? Are they traitors and should be jailed because they fight for a foreign army?