Good question, indeed. Since land tenancy is essentially a rival good (i.e. if A has it, that precludes B from enjoying its benefits) - the whole thing boils down to the means of resolving conflicting claims. If we accept that anyone has the right to land tenancy regardless of the wishes of the current tenants, we basically revert to the law of the jungle - he who can push over the other claimants has the right to the land.
Undoubtedly, many land disputes were resolved in such a way - but would you want to recommend that as a general principle? Would anyone like being kicked out of her land by, say, a developer assisted by armed thugs?
A peaceful resolution of conflicting claims is the cornerstone of civilization. Once we recognize the right of current occupants - we cannot simply push them over to make room for others.
Another question, raised by Yoshie is the right of the state to land tenancy. She seems to argue that because the state is capitalist and imperialist - its right is usurped. I beg to differ, not every state is capitalist or imperialist - yet every state has the ultimate control of the use of the land under its jurisdiction. This is the fundamental premise of state sovereignty. A state that does not have that control is not sovereign, and thus not a state in the ordinary meaning of the world.
The sovereign right of the state to control the land under its jurisdiction is an a priori category that is necessary to understand all other legal concepts including that of the law itself. Its a priori character means that it cannot be justified or explained by other categories, yet it explains and justifies all other legal notions, including that of the law itself.
W may criticize how that sovereign control is exercised (i.e. whether the state should be more or less restrictive in controlling population movement), but we cannot question that control itself without undermining the very notion of human civilization.
PS. The numbers I quoted in my previous posting refer to population densities of different countries (persons per square kilometer) and come from the website a link to which was posted. I do not have the actual immigration figures handy, however, I rely on a relatively recent NPR report saying that EU accepts larger number of immigrants than the US. They did not give the numbers nut I am pretty sure somebody checked that.
The question of European colonialism - the argument certainly was not the "white man's burden" but rather that it was not any different from other similar conflicts between groups and civilizations in human history, the vilification of the North that plays an substantial role in the identity politics of the South notwithstanding.
Wojtek