If someone wants to block certain sites there are client-based solutions that people can set up themselves and proxies they can use if they really want to depend on someone else to decide what they should be allowed to see. There's no need for it at the ISP level. At the very least it should be opt in, but something you have to ask to have removed.
If your mail service had a "make sure mail from known pornographers doesn't get delivered to my house" option, and you had kids at home, you might well opt for it.