

You already need to have a reason to carry around a large blade. Anything over pocket/eating size. Someone with a kebab knife would have to be selling, gifting, repairing or travelling from or to a kebab place.
With these swords, collecting them is a good reason that would make it difficult to prosecute someone carrying it as a weapon. Banning them means the person carrying them is without doubt a criminal, because they are now illegal.
Most people committing knife crimes don’t have a profession (that they would tell police about) that requires large blades.
They are responding to crimes being committed. This helps, and minimises the scope of the law. But, it doesn’t solve root cause.