while cheeky, the above response is dead on. UPB is not any type of argument, it is a framework for establishing the validity of potential ethical theories.
1. Ethics is not a science. The only interests are those of individuals, in particular those of yourself.
2. Economics is a science.
3. According to your understanding of your interests and your understanding of economics, you can support certain norms and institutions.
4. As such, allowing homesteading of something scarce by another person makes sense if you take the increased productivity through the division of labor into account.