You're one of my absolute favorite people on the site, so please don't take this the wrong way. This isn't even a big issue for me, I just wanted to add my thoughts to the mix.
To answer your question, I do see your point, and I think it's a fair one. One thing you should know about me though, just because the other laws are on the books already, doesn't mean I am okay with them.
I'll just give an example, you are forbidden in the U.S. to sell raw milk to people, even if they specifically request it, and have health problems that don't allow them to drink the regular kind. However there is no law forbidding anyone to sell milk with recombinant bovine somatotropin in it.
That being said, I don't think the mail regulation amounts to censorship because you are being sent a physical item by post, that you then have to dispose of in some manner. Also, I can receive adult material by mail if I send a request for it, which is basically what someone does when they go to a website, they send a request to download that page onto their browser.
I must say, from there it doesn't sound like a big step to then put the block in place, does it? I can request the information by opting out of the block, so what's the difference? Here is a sort of counter-point to the original article:
Adult content filters can't replace good parenting | Technology | theguardian.com
Also, I can't seem to tell for sure from a quick internet search, but I don't think there is a law preventing you from viewing adult material on TV? I don't have cable TV, so I get the 6 or so channels that come over-the-air, which are regulated by the FCC. But as far as I can tell, the cable provider offers a parental blocker that you can turn on and off, no government involvement. For the over-the-air channels, I think the FCC does an extremely poor job of regulation, with their double-standards and vague rules. I could dig up stories if you want, I recall a semi-infamous one involving U2 frontman Bono using obscene language with impunity.
I think the free-market solutions are out there, you can get content blockers installed at no charge. Like I said before, I do feel empathy for those who are not tech-savvy, and don't know how to install the blockers. This law may very well help them. And I by no means find the law an egregious violation of my rights, I just don't think it's what would help us the most.