Actually, that's not entirely true. While it is true that there is no compulsory law to accept special needs kids, there is a mechanism within IDEA itself to address this issue. In short, the children need to be evaluated by local agencies. Then a meeting is set up with the school, parents and evaluating agencie(s). At that point, much like what occurs in a public school, an IEP is drawn up and funding is directed to the school for accommodation.
Yes, the school technically has the right of refusal, but I've never heard of a school doing so. Also, there are many, many private schools specifically for special needs kids.