Region Overflow

Can somebody explain this phenomenon to me? When playing after like 5pm on any day of the week, my games average having 7/10 players from China/Korea. From what I understand, they play on NA because their accounts were banned in their region and they can't obtain another one. That being said, they're usually ALL toxic and only speak to flame. Since they were banned in the other region and have 0 devotion to an NA account, they also have no issues with giving up/feeding/while flaming you.

Why is there such an influx of toxic players from other regions? Is there any way possible I can limit my matchmaking ping like in other games to only be paired up with people playing in the same region as myself?