We do need HTTP because sometimes public WiFi networks need you to agree to terms before any requests stop being redirected. I recently found http://neverssl.com
That being said those public WiFi’s shouldn’t be redirecting sites in the first place because for HTTPs sites browsers don’t even let you see the page.
I personally prefer http://example.com as is it is explicitly http-only, adminitred by IANA.
It boggles my mind that we haven't yet agreed on a signaling mechanism at the AP level (DHCP?) for signaling captive portals, as this seems to be quite a common use-case.
That being said those public WiFi’s shouldn’t be redirecting sites in the first place because for HTTPs sites browsers don’t even let you see the page.