You wrote this in the passive voice; it doesn't say who is doing the blocking.
Pornhub itself is doing the blocking; it uses geolocation and denies services to IP addresses from jurisdictions with age verification laws. The laws are usually not structured so as to require a third party such as an ISP to block noncompliant sites; instead, the governments of the states with those laws can sue the porn sites and their service providers (Verisign in the case of .com domains).
> The explicit tube site Pornhub is now blocked in 25 U.S. states
I had assumed that the states were blocking Pornhub but reading between the lines in the linked article it does imply it's not the states are not applying technical blocks.
The states have applied intentionally onerous requirements onto these sites with full knowledge that would most likely not comply making them de facto blocks. You wouldn't be fooled if a gangster said "that's some nice kneecaps you got there, it would be a shame if something happened to them" so I don't know why we are acting so naive about Texas and co.
https://mashable.com/article/pornhub-blocked-states-2025