First you must understand that the U.S. Constitution in Art. I, Sect. 10, affirms our unlimited right to contract as long as we do not infringe on the life, liberty, or property of someone else. That is the basis of all trust law worldwide -- not just in America. The Hale vs. Henkel case merely affirmed this. So first the Constitution says it; then what many call the "Most Important Supreme Court Case" further confirms it. The relevant quote from it is:
"The individual may stand upon his constitutional rights as a citizen. He is entitled to carry on his private business in his own way. His power to contract is unlimited. He owes no such duty [to submit his books and papers for an examination] to the State, since he receives nothing therefrom, beyond the protection of his life and property." (See https://www.scribd.com/document/88506878/Most-Important-Supreme-Court-Case-Hale-v-Henkel)