For better or for worse, the public school system is supposed to teach all children. So a little fellow with a 40 IQ is just as entitled to an education as the one with a 160 IQ. It's just how the services are delivered that will be radically different.
Again, this is not a result of NCLB, but IDEA.
All American children deserve an education. It's not that "little guy's" fault that he was born with an IQ of 40. He's a human being. If special ed teachers were able to teach these students adequately, then parents would not have pushed for mainstreaming them. At least when they are mainstreamed they are visible and people have to pay attention to them - they are not just "baby sat" for 6 hours until they go home. Unfortunately, this is disruptive for the other kids in the class. And if you want to carry the argument further, "normal" IQ kids slow down and are disruptive to high IQ kids who are forced to share a class with them. In addition to piling more homework on high IQ kids teachers commonly use them as unpaid laborers tutoring the normal IQ kids in math or science right there in the classroom, instead of devising lesson plans that meet the needs of all the kids in the class.