How? It's worth asking the question and doing the experiment, have you ever tried making a bioweapon?
For most of us, that's not going to be case, so the next best thing is to read accounts and reports of bioweapon programs. These things are... messy. And expensive.
As a very bad analogy, this is the biology equivalent of saying that I can make a nuke because I bought a drill, a screwdriver and a $200 centrifuge.
Or, saying that I can make Google because I opened MacOS' terminal.
Some people have jobs that are dangerous and necessary to save lives. Others have jobs that are dangerous and involve killing other people.
The real risk will emerge from malcontents in these jobs and positions.
For most of my life, for all its faults, the US Military has been one of the most responsible and competent organizations in the world when it comes to WMDs (and even they lose nukes see the many broken arrow incidents).
But despite some of the most thorough security clearance processes in the world, the only successful terror attack involving bioweapons on US soil was done by a disgruntled / insane researcher at Fort Detrick. https://en.wikipedia.org/wiki/2001_anthrax_attacks
That's the real risk, and let's be very honest here, do you think Anthropic and OpenAI are going to put a filter on the LLM they sell to the US Military?
The other case is Aum, and they fucked up deployment, btw (again making a bioweapon is harder than you'd think). And as I have detailed elsewhere, Anthropic and OpenAI would have sold them licenses too! https://news.ycombinator.com/item?id=49092899
They will say they will have audits in place, but the US Military had those in 2001.