Like Jillio said, this research have nothing with how hard it's to learn ASL do to, but what surprising low levels of fluency a child need in the parents compared to what you, Loml and NCSA claims. The myth from NCSA that a deaf child needs deaf parents to become literate and bilingual is completely false as we see in this research.
Many parents can tell you that ASL really isn't hard at all, and actually a joy to use. What is common is that parents try AVT, Cued Speech, oral ed, mainstream programs, and when that fails, the send their child to a TC or bilingual deaf school. That's were the half of parents with low level skills in ASL come from. Those parents who don't bother learn ASL, don't bother about Cued Speech either.
Where AG Bell leave BiBi alone, and gives parents a different choice, NCSA actively contribute to stop bilingual education by trying to convince parents that they sho:uld not use ASL as a natural/first language with their deaf child. The rudely part, is that NCSA begs to be a part of bilingual programs, who have experience with, and help parents with early ASL. This makes NCSA much more hostile toward ASL and bilingual education than the dreaded AG Bell, to me.
Somewhere in this mess, NCSA is doing the dirty work for AG Bell.