Why are some parents strict over dumb stuff?

[Kristian]

Examples: A teenage girl wants to wear thong but her parents won't let her. (Girls wear thongs at that age to get ride of panty lines, not to be sexy.)
Bedtimes (Why put a child, let's say 6 years old for example, to bed at 7?? Its still daylight outside in most places)
Going out with friends ( Don't deprive them of having fun. They are only young once before they grow up and have worries and stress)
Grades (Unless you're child isn't trying, why punish then for a bad grade. Most likely they are trying and are just struggling.)
Cellphone (What is wrong with having one? They don't have to be on it all the time, but it's a good way to talk to their friends or you.)