It is part of the social contract that citizens defend their borders, and taxation is spent on those with a legal right to be here, those whom voters choose to share their country with.
It is part of the social contract that we pay our taxes so that the State defends the borders so that the country is shared with those the voters wish to.
Think of it the way you\’ve put it here for a moment: "citizens defend the borders"? You mean that the BNP should go around checking on whether non-pinkish people have the correct papers? No, clearly not.