Ah... now I see the point you were making.

I would think that this isn't enough of a threat for AV companies to worry about -- they've much bigger fish to fry. If they spend too much time on 5000 bot networks, they will end up missing things like Melissa. Perhaps more R&D on heuristics would be a better way to go rather than identifying by signature.

Either way, I would also say that it's a user's responsibility to make sure this doesn't happen, regardless of the AV software. Nobody blames McDonnall Douglas' avionics program when F14's get blown out of the sky. They blame the enemy or the pilot.