True but spiders do kill the occasional traveller and might have a few gold near their web from a previous victim. The spider doesnt keep it per se but it is near where it keeps it's victims. That's just an a thought, I don't think spiders should carry gold because this isn't PnP where you would find something.
Also to Crunch, I think the hollow wraith spider is not a normal, natural spider and that's why it might carry treasue.