It's part of the HSTS spec that a server receiving a request over HTTP should redirect to HTTPS.

I assume the logic here is that as it's best practice for any site with HTTPS to use HSTS also, all HTTPS sites should not be available over HTTP apart from a redirect to the secure version of the page.

Not yet, as there was a bit of an argument in the HTTP/2 WG about that.

I may suggest a SHOULD NOT again, however.