What about the possibility that a bad person uses the hammer to build a sarin gas bomb, uses it to destabilize the government, seizes control, and then executes entire races and gene pools? It's all about the consequences, not the probability, so doesn't that make selling a hammer just as wicked?

The US government routinely monitors purchase and search histories to prevent the active ingredients of something destructive like that from falling into nefarious hands, and you're posing a wholly disingenuous argument

Gp isn't remotely disingenuous, they are pointing out that 'probabilities ... are irrelevant [w]hat matters is the consequences' is a philosophy guaranteed to lead to stupid decisions.

To make a sensible decision in a risky context both probability and consequence are needed. There is always a (practically 0%) risk of any action leading to catastrophic global consequences.

In addition, the original Precautionary Principle paper linked is an idea that sounds nice but is actually a bit dangerous. If we applied the PP to the formation of the internet it should have been suppressed right from the get go - we never had and still have no positive evidence that social media won't cause extreme harm to the public (eg, it could have caused mass depression and disengagement with reality and maybe moral corruption :p). The internet is risky - did huge damage to existing industries and triggered the rapid rise of China through knowledge sharing and enabling remote management of industry (why else have we never seen anything like it before?). Impact of research into splitting the atom - did a lot of damage, much more risk.

Proving that something is safe without just doing it is hugely expensive. Technological advancement would not be a fraction of what we have now - the cost of trying things needs to be cheap to encourage progress. We couldn't prove most of the things we do are safe without being able to point to them and say 'seems to be working?'.

Not only that, the precautionary principle applies equally to inaction.

Should we research nuclear physics? The worst that could happen is global thermonuclear war, so forget about it. But what's the worst that could happen if we don't? We might need a nuke to divert an asteroid. We might need nuclear power to prevent climate collapse. It could save the world rather than destroy it.

"What you don't know can't hurt you" is contrary to evidence.

In that case, I would definitely like to see some facial recognition technology so its easier to catch that guy.

If you were going to commit ethnic genocide, how much help would facial recognition software be? Even birth certificates would be more helpful, if the problem was identifying your victims in the first place.