"One is never "free" to violate the natural law"
Perhaps free is the incorrect word to use.
Does one have a choice to violate natural law?
That would be a more accurate wording of what I meant to say.
Of course, man is given free will and God will never tamper with that. We are free to reject Him however justice demands that there are consequences for doing so.
I think of it this way. Say you have a small child, you tell that child "don't play in the street". We articulate the rule to the child for their own health and safety however if we are not going to keep the child on a leash every second she is outside there is the possibility she will disobey your rule. If she runs out in the street bad things can and likely will happen, this was the reason for the rule in the first place. Of course attentive parents will do everything in their power to ensure nothing tragic happens.
God has given us the scriptures, the Holy Spirit and His Church. He became incarnate man to pay for our errors and spoke to men directly to further guide us to the Truth. Nonetheless man still plays in the street.
It's not a perfect analogy, too simple, but we can start there. As I said, start with the encyclical I linked.