Note the stance.
Of course, that specific pic was chosen to show ferocity. (I'm not even sure if that's a FWS, but that is a good example of why they are dangerous)
They are aggressive!! We have a deadly spider hear as well, but it prefers to hide - in fact the Indians who live with them in the woods call it "The Spider That Hides" in their native language.
We also have a very aggressive species of spider here, known to capture and kill birds, mice, etc, called a trapdoor spider. Extremely aggressive, but their poison is not even slightly dangerous to humans. Hurts like fuck to be bitten (seen it, but never been bitten - biting insects and arachnids do not seem to have any interest in biting me, stinging insects and scorpions have no such qualms, however.) and makes a mess of the local skin for a couple of weeks, but only leaves a small scar and no one is known to ever have the kind of complications from this spider bite that can cause death.
It is the effectiveness of their poison against so many species, along with their inherent aggression that makes the FWS so dangerous.