We must buy different bit sets. I go through a lot of dewalt bit sets in all shapes and sizes of plastic yellow boxes and I have never seen a security torx bit included. What bit sets do you purchase?
I have a handy little computer repair set made by Tekton[1]. All its Torx bits are security ones. (note they call them "Star" and "Tamper Star" for trademark reasons I suppose)
http://www.amazon.com/dp/B009MKGRQA