It is the law that all children get a chance at being mainstreamed. No one is being FORCED. You have got it backwards.
It's called the The least restrictive environment ( LRE) according to the Individuals with Disabilities Act.( IDEA) It's cheaper to have a separate class and parents have to fight to get services to provide mainstreaming but the law is on their side.
It wouldn't matter to you that I have been a special ed TA for the last twenty years because you are always right even when what you say flies in the face of law and reason.
http://www.wrightslaw.com/info/lre.osers.memo.idea.htm