"Today, Congress holds hearings on the first American Internet censorship system. [...] The government can order service providers to block websites for infringing links posted by any users. It becomes a felony with a potential 5 year sentence to stream a copyrighted work that would cost more than $2,500 to license, even if you are a totally noncommercial user, e.g. singing a pop song on Facebook. Thousands of sites that are legal under the DMCA would face new legal threats. People trying to keep the internet more secure wouldn't be able to rely on the integrity of the DNS system."
The improbability of persecuting trivial crimes is not the point. The opportunity to provide any mechanism for third parties to summarily censor and sentence others is the definition of vigilante law, and it is bad.