[iforgotpassword]

Sooo, you're behind a captive portal which blocks internet access. So you access an https website on the internet that will redirect you to http. Wat.

[JoshuaRogers]

The browser cachable bit is critical here. By having visited the page before, even though the network won’t allow traffic yet, the cached page performs a redirect to an unsecured page that the local captive portal CAN intercept.

[yardstick]

What Joshua said, plus some hotspots also whitelist some IPs, allow some HTTPS temporarily. The world of hotspots is a real mixed bag on how they do things.