They do because it's true and that's exactly what the law says.

Digital Economy Act 2017 14 (1):

>A person contravenes this subsection if the person makes pornographic material available on the internet to persons in the United Kingdom on a commercial basis other than in a way that secures that, at any given time, the material is not normally accessible by persons under the age of 18.

Section 23: Regulator’s power to require internet service providers to block access to material

(1) Where the age-verification regulator considers that a person (“the non-complying person”) is—

(a)contravening section 14(1), or