alt Hacker NewsTrouble is, that assumption hasn't always worked out in the past: https://en.wikipedia.org/wiki/Bruce_Edwards_Ivins
But yes, this is stupid. No one who is inclined to build a biological WMD is going to be more than mildly inconvenienced by supply restrictions. DNA isn't plutonium.
Sounds like a fully general argument against any sort of security measure. "A dedicated thief won't be deterred by a lock or burglar alarm." Sure. I'm still buying that stuff.