If it is opt-out, it is censorship. This was established through heavy lobbying and threat of legislation. And then there's the one where all major ISPs block an unpublished list of websites that are deemed "illegal". That one was established by court order, and the only way you can opt out of it is by switching to a small ISP.

Here's the blog of one such small ISP, which is well worth reading: http://revk.www.me.uk/search/label/CENSORSHIP