I'm not so sure about that. In Spain we have an example of a country that both receive division-sized forces of volunteers (the CTV, the Portuguese Viriatos, the International Brigades) and sent them (the Blue Division).
Casting our net a tiny bit wider, we have the UN command of several hundred thousand men in Korea, as well as the Chinese People's Volunteers who were more than a million strong. A bit more widely and we have the Chinese in Indochina and Vietnam. Wider still and we have the German expeditionary forces in the Finnish civil war.
Regarding Japan being involved in the SCW and the consternation it has caused:
I should point out that, regardless of how silly it looks, I am more interested in
why countries are allowed to send help than what countries are actually doing it.
For example, Mussolini's massive intervention boosted his popular support at home because of anti-Catholic behavior among the republicans. In this case, a huge intervention wasn't just about generating XP (which is why players will do it), but all about boosting the regime's popularity and cementing ties with a potentially friendly Spain.
Italy is a great example, because intervention meets political goals as well as military goals. France's non-intervention is the opposite; due to concern about the impact on domestic politics, France refrained despite sympathy with the republicans among some of those in power.
I bring it up, because I am not bothered by Japan's intervention so much as I am about the potential for abuse. What does Japan have to gain from intervention in the SCW, regardless of what is happening in China? (Or am I being dense, and Japan's intervention does forward her national goals and agenda?)
What I'm hoping isn't that Japan never intervenes, but that interventions happen with clear goals that make sense beyond "I need to farm military XP."