It would be exciting to adopt same rule as japan. In this case other asean/asian country will have a chance to play in high level of volleyball. As we could see in world ranking European and american dominate the world ranking, asian except china like korea, japan and thailand they are at lower rank. Allowing other asian players to play in this kind of league will improve the skills
For one of asian volleyball powerhouses, Korea actually has really small no of pro clubs compared to others like Japan or even Thailand. They don't even have 2nd division, if I'm not wrong.
So I can understand why they allowed only one foreigner, asian or not, in a club. I think they wanna give more chance to local players.