Nonlibertarians are often puzzled as to why libertarians accept such strong property rights claims (sometimes called absolute property rights, though Ive never figured out what absolute is supposed to mean in this context). The answer Im going to give here is one Ive already offered elsewhere (see here and here), but I want to try out a new way of putting it. (I oversimplify a bit here by not discussing the way in which consequentialist considerations play a role in defining the contours of deontological rights-claims, but sufficit diei.)
The reason libertarians accept such strong property rights claims is that, ironically, a) its much harder to justify rights to external property (i.e., property beyond self-ownership) in libertarianism than in almost any other moral or political theory, and b) because of this, property rights claims have to be extremely strong in order to get justified in libertarianism at all.
Both halves of this claim will seem paradoxical. How can I say that libertarianism makes it harder to justify property rights than other theories do, when everyone knows that libertarianism is the most property-friendly theory on the planet? And how can I say that libertarianism makes stronger property claims easier to justify than weaker ones, when everyone knows that the stronger a claim is (in the sense of being more demanding), the harder (not easier) it is to justify?
The answer to the first question is that property rights claims, like all rights claims (at least in the sense of rights that prevails in political theory), are claims to the legitimate use of force. If I say that Wilhelmina has a property right to her shovel, I am saying not just that it would be morally wrong to deprive her of it, but also that it would be morally permissible for her (or for someone acting on her behalf) to use force to prevent anyone from taking her shovel.
To justify a property rights claim, then, is to justify a claim to the legitimate use of force. And this is generally much harder in libertarian theories than in nonlibertarian ones. After all, nonlibertarians typically endorse the use of force in order to promote all sorts of good causes, so if property rights turn out to be conducive to such causes (as in fact they usually are), then theyll be fairly easy for a nonlibertarian to embrace. But for libertarians, a basic respect for other peoples moral agency implies that no use of force is justified unless in response to someones initiation of force. Hence in order for a scheme of property rights to be justified in a libertarian framework, it must be shown not just that violating such rights has bad results but that violating such rights counts as initiatory force against some person a much higher bar.
And this in turn means that under libertarianism, rights to external property can be justified, if at all, only as an extension of the right of self-ownership. Only if imposing your will on my stuff counts as (indirectly) imposing your will on me will I, or my agent, be justified in using force to stop you. But by the same token, if external property rights are an extension of self-ownership, then they will have to be much harder to override (assuming self-ownership is hard to override) than if they derived from some less sacred source. Hence libertarianism has to endorse either very strong property rights claims or no property rights claims at all.
(Why not the latter option? Because if there were no property rights of any kind private or otherwise, strong or otherwise then it would never be permissible to use force to prevent someone from appropriating any physical objects, which in turn would mean that I and my gang could with rightful impunity starve you to death by seizing all your food as quickly as you found or produced it, so long as we didnt actually touch your body while doing it. But this seems an unreasonable position for anyone, and particularly a libertarian, to accept. So if the no-property-rights option is closed off, only the strong-property-rights option is left.)