It shouldn't be.
All border checks should be abolished. All passports should be abolished. Free people in the world should be able to travel between countries freely, and without having to justify themselves or ask permission, just as they can travel between cities within a country freely.
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.
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.