I repeat the presumption of innocence and being innocent until proven innocent means that McCanns or anyone else for that matter are not obliged to rule themselves out. The police are obliged to do their job with or without co-operation from suspects or anyone else for that matter.
Well, yes ... but the fact that it is not necesssary to prove innocence does not usually lead to innocent people making no attempt to do so
Suppose, for instance, there had been a hit and run crime local to you, and witnesses confirmed that the car involved was the same model and colour as yours. The police come to see you to question your whereabouts at the time the crime occured
You would tell them wouldn't you ?
You wouldn't 'have' to of course ... you could tell the police that you are not obliged to prove your innocence and that it is up to them to eliminate you from the enquiry without any cooperation or assistance from you
But you wouldn't do that ( no innocent person would ) because whether or not we are 'obliged' to prove our innocence, it is human nature to
wish to do so
Innocent people are not expected to be evasive and uncooperative ... that is what is expected of guilty people