Because the courthouse is a limited access area, and you are only giving up your gun for a finite period of time... just while you are in the courthouse.
When Congress limits "some guns" it is forever, for everyone, for anywhere.
and, if you think that Congress would only limit "some guns" once they get rolling, you weren't paying attention in school.
Back in the 70's, 80's the push was to eliminate/take away the terrible "Saturday Night Specials"... another made-up classification. They even wrote songs about it.. then it was "high capacity magazines", and then the "assault rifle"... spend some time and research what our government classified as "assault rifles".... it included .22 autoloading rifles, hunting shotguns, autoloading pistols... thankfully, that insanity didn't last.
THAT is the problem... when the government gets a taste, they want the whole thing....
Why do you think that Japan did not invade the United States after Pearl Harbor?