And if 30,000 camouflaged, balaclava-clad Russian speakers happen to cross the border from Russia to Ukraine, they're just exercising their right as free people :)

Seriously though, it depends on what responsibilities you think the government has towards the people. If a country with high taxes and generous public services bordered a country with low taxes and minimal public services, you can see why problems could arise if people could cross between at will.

What happens if 30,000 camouflaged, balaclava-clad Russian speakers happen to travel from Moscow to St. Petersburg?

I don't see the problem.

If they start shooting at people, then that's a crime and you should arrest them for that, regardless of whether you had any border checks.

And even if you do have border checks, if 30,000 people storm your border at once, a handful of guys with truncheons and paper forms aren't going to be much of a deterrent.

I agree! Also, let's take it a step further and abolish all door locks as well! Free people of the world should not have to ask permission to enter a house!

I'm not saying it's OK to go in arbitrary people's houses without permission. I'm saying it should be OK to go in arbitrary public places without permission.