Just like the people that wouldn't divest in a company that uses slave labor or busts unions: They don't give a damn where the money comes from, as long as the money comes. How fucked up do you have to be to think "Hot damn, a massacre! Time to buy up stock in gun manufacturers!"
I really wish we could've gotten gun control in steps, instead of trying to pass a larger bill with multiple things. If we could've gotten a quick ban on the sale or transfer of large cap magazines the gun industry stocks would probably be tanking.