These pages are unfortunately getting special treatment from quite a few networks these days, among other things to avoid iOS users getting stuck in a loop of trying to connect, and getting disconnected by the OS due to a "non-working connection" for local-only networks like in-flight entertainment systems.

It's a shame that we don't have any standards for the concerns of canonically testing Internet reachability and for authoritatively redirecting network users to a captive payment portal (without having to resort to ugly hacks that often break with TLS).