It states in the above link that if the parents are regarded as suspects they have the right to remain silent and have legal representation, and that some witnesses ask to be named suspects so they have the above options.
If the McCanns claim they have nothing to do with Madelaines disappearance and have asked to be named as suspects, if they have nothing to hide then why would they ask to be suspects so they wouldnt have to answer questions and have legal representation.
IMO I dont think they asked to be suspects, I think the DNA tests have shown that the McCanns were involved in some way.