Did the instructions for the bars say to lubricate the threads? If not, then it was reasonable to torque the bars without lubricant. Do all of you lubricate the threads on all your bolts that you use a torque wrench on? I doubt it. We've all seen guys at the LBS who do this for a living adjust bars and seats for customers without a torque wrench.
I think some of you guys are looking for ways to make this the fault of the installer instead of a fairly fragile part. As I've said before even a top notch manufacterer will make a bum part once in a while and it sounds like the OP got one. It doesn't matter that the MAX torque spec was 5 and he could have gotten away with 3 or 4. If the bar cracked before 5, then the bar failed.