"I asked the reader to imagine that some technology was discovered which could be implemented by anyone with a few million dollars and access to a few good physicists (or perhaps biologists in the more general version I ended with), so in order to answer my question, you have to conform to my premise. "
The technology was discovered hundreds of years ago, and anyone with £5 and an alcohol lamp can implement it. I have implemented it, out of curiosity. Details of the technology are widely distributed in the public domain. True, it couldn't have ended life on earth, but if I was insane I could have made a primitive WMD. And I didn't even break any laws.
You should also bear in mind that if you take ten terrorists and give them ten knives, then the threat from such knowledge pales into complete insignificance.
I totally understand the opinions of many, who see such websites as inappropriate, but they must realise that their existence changes nothing. As Samosa said, all the information is freely available anyway. It's just nice to be able to chat about your hobby with like-minded individuals.
Most of what we discuss is totally useless to potential terrorists, anyway. It's too complicated and too expensive. Remember, the world trade centre was smashed to the ground essentially using craft-knives. Terrorists don't care about novel tetrazole-based primaries, or how to add 3% to their yield of RDX.
The only people that such knowledge will hurt are the people who use it. I myself still have several pieces of shrapnel in my hand from when I was learning the art, and I'm still doing it. Why? Because it's fun
. Everyone likes fireworks, and you can make them much bigger and better if you make them yourself!
Unfortunately, due to the content of the site, it does attract idiots, but we try to ban them as soon as possible. We are at least only sharing the knowledge with people who have enough common sense to use it relatively safely.
I'd like to add that I am currently studying chemistry with biochemistry at Cambridge university, and I certainly would not be there if it were not for the fact that I started playing with pyrotechnics at an early age.