The government should piss off and not interfere. The stations can decide whether they allow "bad" words or not, depending on who their audience is. They time of day should not make a difference. So someone sang piss, shit or fuck or some other word - why should we give a fuck?
If parents don't like oh well if they don't want to listen then they can just not listen at all. Radio stations can pick their music all they want because people like listening to it but if some people hate is so much then they can turn off there radios in there car.
Kids can be listening to the radio and hear cuss words and may think it is okay to cuss in public or in private. Their have been many cases where kids in a school environment say cuss words to teachers or other students who are attending the school thank you.