There are an infinity of possible laws out there which are "common sense" but none the less are not Constitutional.
If my city's library wants to set up computers in the children's section which are censored through a filter, the proper role of the Federal government is to butt out. That is not at all the same thing as saying that the role of the Federal government is to require filters in all library computers nationwide.
Excellent point, N Sojac. The problems come in when some ACLU types sue a local library for doing just that....establishing control over what filters are provided. Then the library, city, county, whatever has to come up with the bucks to fight this case which most of the time, they won’t have. These laws are passed to provide cover for cases like this.
There are no easy answers, but I also agree with your point about the proper role of Federal govt.