They essentially have. They can't blatantly ban them for various legal reasons, but there are other methods. By banning the civilian purchase of new weapons, and by requiring tight registration, they've made the supply strictly limited. The prices are now far above what the items are actually worth, and what's left will either be hoarded or gradually wear out, reducing the number available. The "CLEO signoff" requirement is being abused to reduce the ability to transfer them. By strictly registering owners, the few who are allowed to own are clearly identified (easy confiscation later) or face 10 year prison terms. Few people are even aware that it's possible to own them legally.
It's similar to guns in Japan: yeah, they're technically legal, but effectively they're gone.
