Sorry, pianolearner, I thought I remembered from an earlier post you were. I should have said him/her just to be safe. No offense was intended.
I think the risk, need, and design positions are finally clear.
I believe a society can and should prohibit or restrict items and actions that offer a demonstrated and significant risk. The risk must be quantifiable and compelling, and the action taken must actually do some good. This could apply to guns, porn, radionuclides, drugs, etc.
Pianolearner on the other hand believes it is perfectly reasonable to ban anything that is not needed, whether or not it offers risk. Further, he will also ban anything that doesn't meet his moral standards of design, whether or not it offers risk.
Assault rifles - well, let's call them scarey big guns, there isn't really a good definition - offer a test case. They are involved in a negligible amount of crime, so low as to be almost undetectable. The converse to that is that removing them all from existence will not affect crime rates in the slightest. So they do not meet my standards for prohibition. There is no real risk, nor any benefit from the ban. Pianolearner will ban them because he doesn't know of a need. I could argue that point - pretty much all high power rifle competitions are shot with versions of them - but I won't because I don't admit need as a criterion. Further, pianolearner will argue that they should be banned because they may have been designed with the idea they would be used on humans. Again, I don't see any relevance of design. The fact is they are NOT used on humans to any measurable degree, hence pose no risk.
I will agree that there is a case where "need" does apply.
You can have a product that is intrinsically hazardous but essential. Then there is at the same time justification for prohibition, but requirement to use. An example might be hydrofluoric acid. It's really nasty stuff, you don't want to be anywhere near it, but there are industrial processes that can't be done without it. The reverse case, of something that is harmless but should be banned because it isn't needed, would not exist in a free society.