Kate did decide. She knew that if an innocent person was in prison then the guilty party would still be at large and no-one would be searching for her daughter. Therefore on the advice of her lawyer she took the route that minimised the possibility of the wrong person (herself) being charged - and protected Madeleine's right to continue to be searched for as a living, missing person and not dismissed as a dead one.
You only have to read the 48 questions to know that finding a reason to pin the crime on Kate was the PJ's objective.
Kate's 'crime' appears by some to be that she took the excellent advice of her lawyer. Bizarre.
This has, AFAIK, several assumptions built in.
Is there a source other than Kate saying she was advised not to answer?
Was she advised question by question? Given that she refused to answer some questions but chose to answer others. Who advised/decided which were OK to answer?
It matters not a jot what you think the PJs motive was. Kate had the opportunity to refute any case the PJ made and chose not to do so. Kate had the opportunity to advance the search for Madeleine by clarifying what happened, and she chose not to do so.
It was Madeleine who got a rough deal out of this approach.