A knife is a multi-purpose tool that
can be used as a weapon, and one that has so many important non-weapon uses that not having it would cause far more harm than having it would. This is intended primarily as a weapon, even when used defensively. There's an important qualitative difference there, though it's tempting to gallop down the slippery slope.
Regardless, I'm not proposing to ban either knives or this tool - I've been very clear about that. I do think that - with anything that has potential for harm - it's important for people to consider the possible consequences and to actually engage in the discussion about usage, rather than either washing their hands of the issue or declaring that any consideration will soon lead to arresting everyone for everything.
This is a path we've trodden countless times. With some things, we've collectively decided that no controls are necessary. With other things - poisons, nuclear weapons, a host of things depending on location - society has enacted controls of various efficacy and validity.
The responsible choice, regardless of whether you end up being for or any any given restriction for [thing], is to spend time thinking about it, discussing it, and - particularly when you've chosen to release something - acknowledging the potential issues.