[ghaff]

I think it's the juxtaposition of "US government," "bioterror," and "NYC subway system" (i.e. place with lots of people) that collectively sort of create the expectation that the story is going to be about the government doing experiments that put a bunch of civilians at risk. I think there's also a certain expectation set by the fact that it is a news story that it's going to be a news story about something at least arguably bad that the government did.

I'm not sure that the headline is even clickbaity--don't know how I'd rewrite it. But I can easily see someone reading it to go to the story with an expectation of malice or at least stupidity.