It's a slightly obscure and off-beat line of reasoning that blaming someone -- anyone -- should take priority over identifying true culprits of crimes and establishing what crimes they are (or are not) guilty of ...
Because it was four months after the child disappeared and believe it or not those policemen would be frantic to find out what happened to her. They had the dog alerts, and they had an interim forensics report. So they had a good deal more pointing to the child's parents than they had pointing in any other direction. Four months after the disappearance police can't hang about worrying about the feelings of the child's family.
If you don't believe by the way what Amaral claims Prior said, you should look into the evidence held by police at the moment people have been charged never mind arrested. Not much in many cases, including high-profile ones - in UK and other countries. Check, for instance, the evidence held by the US police at the moment Casey Anthony was indicted, in a case very similar to this one in many respects. Not much - dog alerts, single hair, smell of Chloroform. That was more or less it. No body.