For one, it’s not only Rothbard, but I believe you are talking about a philosophical class of individuals.
Secondly, that’s easy. Because of the starting point or values that Rothbard holds. Rothbard does exactly what Mises does by asserting that economic action starts with private property. Mises asserts that peace is what lead to private property prosperity. Mises asserts that social law is what maintains a society in order to thwart violence against the society. What Rothbard did was compliment Mises on efforts by taking some time to think about private property and how it is defended.
Basically you’re arguing against agent-relative values or what some people, though philosphically primitive word usage in my opinion, like to call subjective values.
I mean are you asking something more? How nitty-gritty is your question?
Edit: I’ve posted this link before to Nitroadict but I don’t know if he has read it yet. I assume not as he’s still bolding questions, meaning, he’s not offering knowledge only still wondering what particular things mean, ie. bolding a question, etc… I don’t know if either of you have read Mises “Socialism” but he writes about ownership at the beginning of that book of his as well.