What libertarians propose is that all coercion be in defense of property ownership. This seems "fair" until you realize that "property ownership" is itself a complex historical object. The feudal system is named after a form of property ownership - the feodum or fief. The colonial system created property for Europeans by appropriating it from natives. The system of slavery was also based on a certain kind of property definition.
Saying "don't use coercion except to defend legitimate property rights" is like saying "don't use military force except to defend against legitimate threats." It seems like common sense, but all the complexity of politics is swept under the rug in these phrases "property rights" or "threats". In my view, no one is born with a "right" to property, property is just a concrete representation of the political equilibrium that exists at a certain time.