I'm not sure I see any such implication here:
This looks like an autonomous volitional act by Asfaloth, with which
Frodo has nothing to do. Only later in the passage does he feel "commanded urgently to halt". It's true that "he had no longer the strength to refuse", but neither does he try to restrain the
horse, who has turned in defiance of the
Nazgul. We could speculate that he
might, had he had the strength, have at least tried to urge the horse to turn and run, but Asafloth seems to have had a mind of his own about it.