This is a repost of something I wrote a long time ago:

Filtering creates four problems.

First, it requires building an infrastructure that can be used for the censorship of any content. A different government elected to power could quickly react to a "crisis" situation by blocking access to international news sources. They could literally move from decision to implementation in hours. So the infrastructure represents a significant threat to liberty regardless of how it is being used today.

Second, it is usually overbroad in practice. The system in the UK seems very carefully maintained and so it is the exception to the rule. But even they ran into a problem where they literally blocked anonymous editing of Wikipedia for the entire country. Typically these lists are not well maintained and sites end up being listed that should not be listed. In most cases these lists end up containing sites that are blocked for politically motivated reasons.

Third, there is a lot of different kinds of content that western governments deem illegal. Gambling, information about narcotics, "hacking tools," torrent lists, etc. Once the system exists there will be political demands to expand its application and there is no clear limit to what might be filtered. Once the list begins to expand the risk of accidental or politically motivated blocks increases.

Fourth, to filter is to surveil. You can't block peoples web traffic without inspecting it. When it comes to something like child porn you'd be remiss if you didn't investigate hits on your filters. So these filtering systems constitute a defacto surveillance system. At least in America you'd have to square that with the principal that you don't spy on people without probable cause, and other countries claim to uphold similar values. I think in the US they'd argue out of this corner using the rationale applied in Illinois v. Caballes - that you've no expectation of privacy in regard to evidence of your guilt, but I don't personally agree with the conclusions of that case - I think it opens pandora's box for surveillance technology.

Child porn is bad, the police ought to go after it, but that doesn't mean that and any and all approaches are equally reasonable. A total internet censorship infrastructure is a step too far in my view. I think the threat that it represents to liberty outweighs its usefulness as a law enforcement tool.

Why I don't think mandatory ISP Content Filtering is a good idea.