Numerous academic papers have been written about this question. As far as I am concerned, the answer is quite simple.
According to the Ramban, everything is מושגח and controlled by Hashem. If a person sins then Hashem's Hashgacha is to leave the person to nature - or at least it will SEEM that he is left to nature even though in fact it is directed by Hashem.
I recall that Rav Dessler and his talmid Rav Chaim Freidlander say the latter.
It is actually a Parsha in the Torah. אם תלכו עמי בקרי אף אני אלך עמכם בחמת קרי. You want chance? I'll give you chance.
