This law is a perfect example of a knee-jerk reaction to media headlines. It penalizes a large number of people who have nothing to do with a problem. Do you remember the football hooligans? Why wasn't football banned then?
This is also an example of law based on presumption of guilt – everybody is guilty until proven innocent. I understand this law is there to prevent me from harming somebody else. Of course I have no such plans or even intentions but my verbal protests are not taken into account. In order to prove my innocence I would have to get myself arrested and appear before a court. In this case punishable is not a plan or intent to harm anybody, punishable is just the ability to do it.
To demonstrate absurdity I can use here an analogy with another tool that might perhaps be banned – the one that some men use to rape women. Are all men to be presumed guilty of intent to rape? One would think that this is an extreme simile but a recent story of a man from Luxembourg who was told not to sit next to a strange boy on a BA plane proves otherwise. Apparently it is the BA policy that men should not sit next to strange children because all men are presumed potential molesters. It is true that these are internal regulations of an airline but it seem that lawmaking goes in this direction.
I would say a blanket law like this is sinister because it reminds me of laws in the country I grew up in – which was communist Poland. I was a dissident there in the 1979s. Freedom of speech was constitutionally guarantied and so I could not be prosecuted for expressing my disapproval for the government but there was a number of laws that made almost anyone criminal. These laws were considered silly and were ignored most of the time even by the police, but they could be used to prosecute a dissident. For example it was illegal to walk the street without an identity document, one could be arrested on the spot for that. It was also obligatory to report to the police if one wanted to spend a night away from home, which meant I could be prosecuted for not reporting to the police when visiting an aunt for a few days.
The blanket ban on knives is similar in one important respect – it enables the Police to prosecute a few and criminalizes a large proportion of the society.