My problem is that no matter how benign a treaty might sound in the 
beginning it would lead to the creation of an infrastructure to allow 
enforcement. Once you have an international infrastructure of control 
who is to say the rules might change? So something might start out as 
the society for the protection of cute kittens organizing to stop child 
porn and end up with the thought police installing chips in your brain.

And you can imagine where this would go when it comes to "religiously 
offensive" materials sent across the internet. There are many countries 
where not believing in God caries the death penalty, as well as 
believing in God the wrong way. I can imagine what would happen between 
Christians and Muslims on an Internet with a central control 
infrastructure. There was a story recently where a man who was a 
non-believer determined that a crying statue of the Virgin Mary was 
caused by a leaky sewer pipe and he's being prosecuted for it. Imagine 
what a threat it would be to realists if those views could be enforced 
across international borders.

And what about uprisings? The Arab Spring was organized online. Would we 
be obligated to censor the cries of the oppressed and tortured because 
of treaty obligations of the oppressing country?

The bottom line for me is that some criminality is the price we pay for 
freedom and it's worth it. Once you put in an infrastructure to stop the 
bad guys then that infrastructure can, and most certainly will, be used 
against the rest of us. So I support our resistance to any treaty or 
domestic law to centrally control the internet.