Yes, because the president wants a bunch of people with questionable skills and itchy trigger fingers playing cops and robbers in the event of a terrorist attack.
Despite what people may tell themselves, most of us aren't John McClain. A bunch of gun enthusiasts with questionable skills and minimal training are no match for terrorists who have trained specifically for their terror mission, who have automatic weapons, and are primed for this action. At best they may cause a distraction (because, come on. It's not going to be like Die Hard) and at worst they lead to more civilians being killed either through cross fire, friendly fire, or just enraging the terrorists and making them shoot more.
The reason the president isn't calling for more citizens to load up isn't because he can't do it politically; it's because he knows that would only add gasoline to a fire in the event of a real terrorist attack.
In closing: I know, I know. YOU'RE different. YOU were in the Military/Merchant Marines/Police (Nah, they'd know how stupid this is)/Call of Duty Leaderboards! YOU would know how to handle yourself in that situation.
But you're just one of the gun owners out there. Who knows what skill level the others are at? Even if we assume that you're John McClain, what about the guy sitting in the darkened theater seat next to you? Or the guy next to him? If there are ten concealed carriers in the next terrorist attack, how many of them do you think will have any applicable skills to deal with the situation?
So now we've got four terrorists with superior weaponry and training--not to mention coordination and communication-- vs a handful of people who happen to have guns but don't know each other. Yeah, they'll be an effective counter terrorist force!
What about the people who aren't carrying? The other civilians in the picture: what do they do when suddenly there aren't four gunmen but fourteen? Do you think they will react positively? They were probably already panicking before, but this will only serve to further destabilize the situation.
What about when SWAT eventually storms the place? All they know is there are a lot of gunmen in there. How do they separate the vigilantes from the bad guys? How do the vigilantes separate the bad guys from SWAT, who are probably coming in guns blazing?
The president isn't asking for yokels to carry guns because it'll only ramp up the already intense danger. I mean, come on.