Doors and fences help you enforce private property rights at the micro level but that doesn't necessarily justify national boundaries.
Conservatives love free markets. That means they love unrestricted movement of capital and labour across the globe according to demand and supply. E.g. the level of wages in Australia becomes too high, the law of supply and demand will efficiently move labour from across the globe to Australia.
If you love true capitalism then you should see no reason for national boundaries other than it adds to the cost of capitalism.
