What’s in question is not whether or not people are entitled to private property. What’s in question is whether or not all currently existing claims and legal titles to ownership are legitimate according to libertarian theory.
Allow Rothbard to illustrate:
http://mises.org/rothbard/ethics/nine.asp
"Thus, we cannot simply say that the great axiomatic moral rule of the libertarian society is the protection of property rights, period. For the criminal has no natural right whatever to the retention of property that he has stolen; the aggressor has no right to claim any property that he has acquired by aggression. Therefore, we must modify or rather clarify the basic rule of the libertarian society to say that no one has the right to aggress against the legitimate or just property of another.
In short, we cannot simply talk of defense of “property rights” or of “private property” per se. For if we do so, we are in grave danger of defending the “property right” of a criminal aggressor—in fact, we logically must do so. We may therefore only speak of just property or legitimate property or perhaps “natural property.” And this means that, in concrete cases, we must decide whether any single given act of violence is aggressive or defensive: e.g., whether it is a case of a criminal robbing a victim, or of a victim trying to repossess his property.
Another vital implication of this way of looking at the world is to invalidate totally the utilitarian way of looking at property rights and therefore of looking at the free market. For the utilitarian, who has no conception, let alone theory, of justice, must fall back on the pragmatic, ad hoc view that all titles to private property currently existing at any time or place must be treated as valid and accepted as worthy of defense against violation. This, in fact, is the way utilitarian free-market economists invariably treat the question of property rights. Note, however, that the utilitarian has managed to smuggle into his discussion an unexamined ethic: that all goods “now” (the time and place at which the discussion occurs) considered private property must be accepted and defended as such. In practice, this means that all private property titles designated by any existing government (which has everywhere seized the monopoly of defining titles to property) must be accepted as such. This is an ethic that is blind to all considerations of justice, and, pushed to its logical conclusion, must also defend every criminal in the property that he has managed to expropriate. We conclude that the utilitarian’s simply praising a free market based upon all existing property titles is invalid and ethically nihilistic."
http://mises.org/rothbard/ethics/ten.asp
“In the present world, when most land areas have been pressed into service, the invalidating of land titles from never being used would not be very extensive. More important nowadays would be invalidating a land title because of a continuing seizure of landed property by aggressors. We have already discussed the case of Jones’s ancestors having seized a parcel of land from the Smith family, while Jones uses and owns the land in the present day. But suppose that centuries ago, Smith was tilling the soil and therefore legitimately owning the land; and then that Jones came along and settled down near Smith, claiming by use of coercion the title to Smith’s land, and extracting payment or “rent” from Smith for the privilege of continuing to till the soil. Suppose that now, centuries later, Smith’s descendants (or, for that matter, other unrelated families) are now tilling the soil, while Jones’s descendants, or those who purchased their claims, still continue to exact tribute from the modern tillers. Where is the true property right in such a case? It should be clear that here, just as in the case of slavery, we have a case of continuing aggression against the true owners—the true possessors—of the land, the tillers, or peasants, by the illegitimate owner, the man whose original and continuing claim to the land and its fruits has come from coercion and violence. Just as the original Jones was a continuing aggressor against the original Smith, so the modern peasants are being aggressed against by the modern holder of the Jones-derived land title. In this case of what we might call “feudalism” or “land monopoly,” the feudal or monopolist landlords have no legitimate claim to the property. The current “tenants,” or peasants, should be the absolute owners of their property, and, as in the case of slavery, the land titles should be transferred to the peasants, without compensation to the monopoly landlords.”