I don't think no-poaching agreements infringe anyone's liberty and it that sense I don't see how it goes against free market principles. If you sign a contract with your employee saying you won't do no-poaching then doing so would be a breach of contract obviously but that would be a specific use-case. I totally agree that it breaks the laws though as it is illegal in the US. I was talking about free-market principles and no-poaching doesn't break these principles.