You know these filters are optional, right? Comparing these to the filters for images of child sexual abuse is bizarre - they're operated in very different ways.
as for the linked content-code, it is true that its is not a industry regulation but a rather a industry practice code written by large ISP themselves.
The Internet Watch Foundation provides a list of sites that are involved in images of child sexual abuse. While they're an industry group, they are a bit more serious about their mission than other filter providers.
(http://www.iwf.org.uk/about-iwf)
I guess they were formed to avoid the need for legally mandated filters.