The intent of the Americans with Disabilities Act (ADA) is to give equal access to disabled Americans in public places. Bathroom accessories, such as towel bars, have placement recommendations to make them easier to access, particularly from a wheelchair.
If you can only approach the towel bar from the front, then the ADA says the maximum height for the bar is 48 inches from the floor. If there is an obstruction, such as a counter, that is more than 20 inches deep, then the maximum height is 44 inches.
If you can only reach the towel bar by moving a wheelchair parallel to it (side access), then the maximum height of the bar is 54 inches from the floor. If there is a counter or other such obstruction, the maximum height is 46 inches.
Grab bars and towel bars are not the same thing. Grab bars help you regain your balance if you slip and so are constructed to be weight bearing. Towel bars hold towels and are not likely to hold your weight.