Its because real life is not black and white. Its shades of gray mostly.
And while its ease to spot and codify extreme cases on both sides, its impossible for a law to clearly codify a clear line where good turns to bad.
The 'original' argument for this type of conversation is: what makes a nude picture an art piece, what makes it porn. If we were given 100 photos we would probably agree in most cases whats art and whats porn, but try to write rules for distinguishing them.
but not a good example to convince someone that privatized censorship is akchally a good thing.