Whats the reason for having to pay money to buy tiles that don't belong to anyone? You're telling me early settlers just payed some anonymous person gold to take territory that was nearby their villages? Who the hell came up with this one..
Although I think gameplay should always come before historicity, I can think of any number of historical analogies for this. The land isn't necessarily unoccupied. It's simply uncivilised. You are paying to make it a land which bows to your culture, which works at your behest and which is home to your armies and people. Perhaps you are bribing its inhabitants to yield it to you. Perhaps you are paying your citizens to move in and occupy it. The mechanic is so simple it could mean anything or nothing. But that's what you get when you try to create a set of simple game mechanics which encompass the nature and structure of human development through the millennia.
I don't really like this mechanic; it makes coinage far too useful.
And whilst thinking about it logically; you can claim that it's bribing the uncivilized or whatever; but it's sort of funnier to think of a divine landlord that owns every square on the map that isn't occupied...