Good logic:
We should do x, because if we do x we will obtain good outcome y or avoid bad outcome z.
Bad logic:
We should do x, because you can't provide a reason I accept as valid as to why we shouldn't.
That second piece? The argument you keep making? Doesn't actually provide a justification for doing x. It is an attempt to shift the burden of proof. It's "God exists because you can't prove that he doesn't". It's irrational crazy-talk. And most people, when given two options with no justification, would avoid that one that involves imposing restrictions on freedom solely to impose restrictions on freedom.
What makes this worse, is that in this case you could easily make the good logic argument with even a modicum of evidence to support the position. It's not actually a hard argument to make. The fact that you'd go to argument Bad Logic is admitting that there isn't a good reason to ban them, but you want to anyway. (Whether or not you actually want to, this is what you are actually communicating when you make that argument)
If that's the justification for supporting the ban, those people who believe it ARE stupid, yes. And probably willingly so, to justify a preconceived belief, since you generally don't accidentally arrive at that sort of argument.
Luckily, I don't think that is actually the justification for most of those seeking it, and that they have a bit of intellectual honesty and are instead trying to make legitimate arguments.
Here's an example of a potential real argument:
"Extended magazines have been used in crimes before and likely will in the future. Evidence indicates while that the crime would have occurred even with this ban, he would have been unable to acquire a magazine and the body account would have likely been reduced. To save these lives in future occurrences, we should ban these."