He's ignoring the point that 'rule of law' didn't exist in the Americas. Maybe private property and rule of law existed in Europe and made for a better society there, but the conquistadors that came to the Americas acted like the rule of law was "whatever I want to do, I get to do." Ideas like private property don't excuse things like slavery and murder.
You can't say, "I claim this piece of land for my own. Oh, look! There are people living on it already. I guess I get to enslave, rape, torture them." This has more to do with brutal people being the first people to encounter the natives in the Americas, and deciding that since there was no European justice system or society to condemn/jail then that they could do whatever they felt like.
"I claim this piece of land for my own. Oh, look! There are people living on it already. I guess I get to enslave, rape, torture them."
Actually, you can say EXACTLY that. The very idea of private property is founded on that principle. When whoever it was originally decided some plot of land belonged to him, that's precisely what he said. All I would add to that statement is the clause, "Or they can get the hell out."
I think he's saying that if someone came into your house and said it was theirs, and had the ability to enforce that, it wouldn't be your house anymore. No, you can't ethically say "I claim this piece of land for my own. Oh, look! There are people living on it already. I guess I get to enslave, rape, torture them." But if ethics are not of any concern to you, then you can certainly say it and do it, and the land will then be yours.
Ok, so that's more "how" than "why". The "why" was because they wanted it.