If Time is Relative, Why Have a Curfew?

I’m sure every teenager has to or had to deal with it : the dreaded Curfew. Despite my attempts to pacify my parents with long winded explanations about how time in the city is actually different than time in the country, it never flew, and I was subsequently grounded. Makes sense to me though – through the laws of gravity relativity time is slightly faster in areas of less gravity and is slower in areas of denser mass and gravity. Thusly when I am in the city, time is slower – there are lots of building that weigh a lot, right? All the while their clock at home in the country is running faster due to the lack of buildings, ya follow. I would leave the city in time, according to my slower watch of the city, yet magically arrive home later than the preapproved time. This is even despite efforts to stretch out my time by following the equation Time = Distance /Speed. I was going faster in my car, covering more distance with more speed, than was probably alloted by law, yet the time was still different. And to every teenage action there is an unequal and completely opposite reaction by the parents. Which leads me to this…is a curfew necessary in raising a child? Is trust misgrounded if a parent allows the teen to go about and not come in at a certain time? Or are curfews as much for the parents as they are the teens?