I bet some of the weight restrictions are to prevent you from suing them if you had an adverse health issue form from the treadmill (ie heart attack)
I am not meaning that to sound harsh either. I just know that rarely do you find a person who is a cross country runner weighing in at higher weights. I also know that anyone can suffer from health issues, heavy or light, but I would think that somethings are just to cover the company's butt.
I am pretty positive, that a walk on a 4 speed will not hurt the treadmill