1. Humans are capable of acting in the praxeological sense, animals are not. Humans have self-ownership, while animals do not. Because of these distinctions humans are actors rather than property, and the reason coercion is bad should be self-evident to anyone who wears that flag.
2. A man's future will may not be alienated. Per Rothbardian contract theory contracts whose violation do not result in theft are not criminally enforceable, including a contract stipulating indefinite servitude.