"......should those same parents be allowed to -- say -- set up a subscription website ..." 

Umh, I think that's where the "common sense" part comes in, though granted it's an endangered species these days,  Unfortunately, the easy solution is simply to create intellectually lazy one size fits all rules and common sense be damned.  There is a world of difference between family photos on Flickr and 'All nude babies all the time only $49.95 a month automatically renewing' Yes, it would be easier to simply ban both just as it was easier to suspend the little girl who came to school with her Mickey Mouse purse that had a chain handle - violated the 'no weapons' policy you see. Easy but insane. Somewhere between the extremes lies sanity.  

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken