This is a common thought-process block which, once you get past it, looks really silly afterwards.
The fallacy goes like this: "You can't make X illegal, because there are so many of them out there that you can't go confiscate them all."
As pointed out in many contexts, the fallacy assumes that there is any need to go out and confiscate whatever it is. That's not really necessary, and it demonstrates that a lot of people just don't understand how this sort of thing works.
The way it works is simple, and it applies to more than just this, so we can generalize it to "thing X".
Thing X is a legal thing which is used in the commission of certain crimes. Society makes a decision, through its elected representatives and officials, to outlaw thing X.
Nobody - not a single soul - has to go out and hunt down who owns thing X.
What we are concerned about is criminals using thing X to commit crimes. And, yeah, as the saying goes "if thing X is outlawed, only outlaws will have thing X."
That's true, and it is very helpful.
Because, here's the thing.
1. Some guy gets pulled over for running a stop light. While asking for his license and registration, the police officer sees a thing X in the back seat of the car.
2. A neighbor reports a violent domestic disturbance next door. Officer responding to the domestic disturbance see a thing X sitting in the living room in plain view.
What these kinds of regulations do is to remove the element of "oh, we'll just have to wait until he tries to do something illegal with it" from ordinary situations and encounters where the thing X might be found on a person during an encounter with law enforcement.
If the accessory, whether commercially acquired or made at home, is itself an illegal device, you don't have people showing up at the range to practice with it, you don't have people openly transporting it, and so on. It becomes something of a nuisance to own or possess, which impacts the extent of its distribution generally.
It's astounding how many otherwise intelligent people think that any "law against X" requires a swarm of law enforcement agents to scour the countryside in pursuit of X. That's not how it works with literally anything else.
I'm off to buy my lawn darts. Ciao.