Earlier quoted context omitted.
Android detects wifi with captive portals automatically and pops-up a notification that says "Wifi network requires sign-in". Clicking on that takes you to non-HTTPS page in a browser that is intended to be intercepted. There is no reason why Apple can't add captive portal detection at OS level like Android does.
As the OP said, that is already in iOS - it is just that more and more captive portals are whitelisting the domains it hits to check for a captive portal.
Why?