Why is the Incorporation so controversial?

I am not saying his critique of Tom Woods is completely correct. Nor do I agree with Kinsella completely either.

My point was: why would an anarchist care about whether something is legal according to positive (vs. natural) law or not? Leaving Tom Woods alone for now (I really do like most of his works and videos and respect him, so I am willing to give him benefit of the doubt re: being an anarchist), here is my question (and I asked the same question in a different thread titled something like “Why is Ginsburg wrong?”):

Imagine Jane owns some slaves. Joe marries her, and they create a marital contract regarding their shared use of Jane’s property. The contract is designed to make sure that Joe does not excessively abuse Jane’s slaves. The contract lists only those things that Joe may do with the slaves.

One day Jane is abusing her slaves. Joe forces her to stop. A libertarian lawyer walking by sadly shakes his head lamenting the abuse of Jane’s property rights. “Now Joe can do whatever he wants with the slaves. Their safety is in question. Joe violated the contract with Jane.”

But 1) isn’t the contract a joke anyway (at least the part of it pertaining to the slaves) from natural rights point of view? 2) shouldn’t the lawyer worry about what’s being done to the slaves at the moment and rejoice at every instance when their rights are upheld (or abused less)?