Besides, it's much riskier driving your car to the mall, than the tiny risk of being shot by terrorists when you're there. Auto accidents kill 50,000 Americans every year (and 250,000 people worldwide). That's far more than have ever been killed by terrorists, but we do absolutely nothing about that.
Of course we do a lot about car safety. Are you kidding me? Traffic laws, speed limits, mandatory seatbelts laws, airbags, crumple zones, government mandated safety ratings, harsh DWI penalties, etc. But there's only so much you can reasonably do to protect people while still leaving that mode of transportation viable and cost-effective.
Besides, everyone understands that accidents happen, and can't realistically be 100% avoided or prevented. They're part of life, and while sad and horrible, it's so much more awful when someone deliberately takes away someone else's life. So, I understand what you're saying, but don't think you can really compare auto accidents and terrorism (or disease and terrorism). It's the vicious and deliberate act of terrorism that makes the deaths so much more shocking than an accident or death by illness.
And also, you're perhaps neglecting the "terror" part of terrorism. It's not really about the number of deaths... it's about the psychological impact they have. That's why bringing down a plane is probably more effective (terror-wise) than randomly shooting people in a mall - because people are already afraid of flying to some degree, and the fear of someone deliberately bringing down the plane only heightens that. Granted, the effect would be heightened at a shooting during the Christmas season as you mentioned. Ugh... I can't even bring myself to understand what sort of mindset it would take to do things like that.