We've got that in the UK. Most (all?) cell service providers require you to call them up and ask them to unblock your access to "NSFW" sites over mobile data.
I recall the Vodafone customer service rep very distinctly asking me, "Sir, are you aware that this will mean you will be able access websites of an adult nature?" and replying with a resounding "Yes, I look forward to it, please go ahead!"
I've no doubt btw that the language and the strident tone used by the rep are meant to nudge (perhaps even shame?) the customer away from enabling this option. Joke's on them!
The UK did not go through with the porn block. You can perfectly well set up an account without any NSFW filtering, and as far as I can remember, it wasn't even 'opt out'. I'm with Virgin--oh the irony--and have never had any filtering.
If it happened to you, perhaps you just happened to set up your ISP when there was _talk_ of the bill, but ultimately it didn't pass.
The GP was talking specifically about cellular (mobile) data. Three[0], Vodafone[1], EE[2] and O2[3] all have an adult content filter enabled by default for mobile data.
