I think, for the most part, it's that people WILL try and be the psuedo-rambo gun-toting characters. The restrictions cut that chance, though people still try anyway. 

Most people, from my experience, aren't a fan of numbers and having to count their rounds and whatnot, so it is usually ignored. again, my experience.

But to be fair, a lot tend to let you pick up guns down the line, as long as it's logical. Since most post-apoc things take place some months-years, guns aren't going to be all that common anyway, ammo included.