"If the intruder were carrying chemical, biological or radiological weapons and President Obama and his family had been in, we would have had a dead president as well as a dead first family."
And if he'd have been an Extraterrestrial Assassin, or a Time Travelling Killbot, or plenty of other really F-N unlikely things...
By the way, the chemical weapons that could be used to take out someone in the building without being in the same room, and possibly closer than that, is going to need something that would have to be concealed in a backpack or the like anyway. This isn't a hollywood movie with their james bond size lighters that gas entire military bases, or their john wayne evershoot guns that apparently carry hundreds if not thousands of rounds of ammo.
All the tiny stuff you can hide in a pocket is going to require you to be really close.
Now that chances of the intruder actually being a threat is actually really small. The odds of him having anything realistically dangerous without a sufficiently sized container to hide it in, like the previously mentioned backpack, is also really small.
Over the decades, there have been lots of people that have broken into the white house grounds. I've never heard even a single one of those reports in the last century being of hostile intent. (Weird and or confused, but not hostile.)
So, with odds like that, you want them to do something horribly over-reactive to make them look really bad and get called fascist nazis by the press, just to make you feel a little better? Not going to happen so long as they maintain even the slightest iota of common sense.