just imagine what will happen if someone does try to run onto the main stage. or worse gets near a microphone.
the entire internet will go nuts over the videos of him being dragged off by security. the streams will either be cut or go to technical difficulties. the crowd will go nuts and the panel thats going on will almost certainly be canceled.
and then what to do you do as blizzard? try to continue as if nothing happened the next panel?
we already know blizzard has increased security for blizzcon, so i'm pretty sure they have considered this scenario :P