How do you know they have refused to do this? Why does the acknowledgement need to be made public? To satisfy whom and why?
I know by reading and listening to what they have said. They voluntarily went public, no-one forced them give interviews, partake in photo-shoots or write a book.
Some people accepted their story without question, others didn't. The McCanns and some of those who accepted their story attacked without mercy those who didn't believe the story, and they are still doing it.
Questioning the McCann's story isn't evidence of madness, badness, sadness, jealousy or sectarianism. The anamolies, evasions and the inexplicable pronouncements are all real and the evidence of them is real too.
In my opinion blackening the characters and questioning the motives of those who haven't accepted the story doesn't answer the questions, it just verifies that those doing it are unable to provide believable answers.