Seventy-something for a diabetic, especially an insulin dependent diabetic, is an entirely different situation than for a non-diabetic.
A person with healthy glucose metabolism responds instantly to changes in BG. There really is no risk of hypo.
A diabetic has, at best, a 30 min latency for insulin reaction, and a long tail (2-4 hours) of continuous metabolic activity. This makes "control" much more difficult. It's like trying to drive the Mars rover directly when the time lag between what you see on the cameras and the control inputs you give in response is 30 minutes.
To avoid driving off that cliff you're going to have to make some steering/throttle decisions when it's way, way off in the distance. Something you just can't see way out there might become a problem in the next half hour, and there's nothing you could have done about it -- you couldn't have forseen it. All the information you were using to drive the thing was 30 minutes old when you made your navigational decisions.
Treating diabetes is exactly the same way. We are always doing our best guessing at the future, and whatever we do to affect BG has a response lag.
So, while physiologically 70's are no big deal for the vast majority of diabetics (heck, people), the issue really isn't the immediate impact of 75, or even 65 on one's state of health. Basically no one is at risk of anything harmful at those ranges. Rather, it is the risk of what will happen in the next 30-60 minutes that's truly critical, and dangerous.
Staying above 80 gives diabetics time to deal with over-administration of insulin. It really has nothing to do with actual physiological harm at slightly lower levels.