Like I said it is just an opinion,I can't speak for every person out there. So to argue with your statement would be wrong of me. In my view I see it as,tackle your fear. Go head on,since a phobia is fear of a certain thing or element. So if the spider is aggressive or not,the phobia isn't a fear of the temperament of the spider but the spider itself. Yet again just me summing up how I see it,arachnophobia is probably one of the most common fears. I asked people what makes them fear a spider -
Either the hairy legs
The beady eyes
The fangs
The bite
The venom(even if its a harmless one)
Walking over or on them

And there are many more reasons,but it stems from one thing fear of a spider. I also think we try and rationalize the fear with the above mentioned reasons,like any other fear.