É explicado no próximo livro.
“So tell me, Harry,” said Dumbledore. “Your scar... has it been hurting at all?”
Harry raised a hand unconsciously to his forehead and rubbed the lightning-shaped mark.
“No,” he said, “and I’ve been wondering about that. I thought it would be burning all the time now Voldemort’s getting so powerful again.”
He glanced up at Dumbledore and saw that he was wearing a satisfied expression.
“I, on the other hand, thought otherwise,” said Dumbledore. “Lord Voldemort has finally realized the dangerous access to his thoughts and feelings you have been enjoying. It appears that he is now employing Occlumency against you.”
Harry Potter and the Half-Blood Prince Chapter 4, Horace Slughorn
Atualizou a resposta.
Para ler os pensamentos de Harry, Voldemort teria que entrar em sua mente, mas ele não pode / não iria entrar na mente de Harry porque ele não conseguiu possuir Harry uma vez (por causa do amor). Isso teria ferido o ego de Voldemort e ele não tentaria fazê-lo novamente.
That power also saved you from possession by Voldemort, because he could not bear to reside in a body so full of the force he detests. In the end, it mattered not that you could not close your mind. It was your heart that saved you.
Harry Potter and the Order of the Phoenix Chapter 37, The Lost Prophecy