Apparently we still don't have a way to download release images over HTTP instead of FTP, and this causes problems - sometimes it's just weird (like, trying to download the image using Safari), sometimes it's a firewall that blocks FTP.
If you look at the output of 'host -t cname ftp.freebsd.org', and it returns: ftp.freebsd.org is an alias for ftp.geo.freebsd.org. then this is already resolved. Not all mirrors have HTTP, in particular the mirrors admins@ do not have full control over.
All of the "ftp.freebsd.org" site is itself a cluster of mirrors. It listens on ftp and http. It doesn't listen on https, but in theory we could do that too. As Glen said, we don't have any control over 3rd party mirrors.
In any case, they do support http, eg: http://ftp.freebsd.org/pub/FreeBSD/releases/amd64/amd64/ISO-IMAGES/10.2/ If this isn't mentioned (or isn't clear) then its a doc bug and could be updated over there.
A commit references this bug: Author: allanjude Date: Sun Feb 5 10:56:15 UTC 2017 New revision: 49926 URL: https://svnweb.freebsd.org/changeset/doc/49926 Log: Switch the download page to HTTPS instead of FTP PR: 200327 Reported by: trasz Reviewed by: bcr Differential Revision: https://reviews.freebsd.org/D3754 Changes: head/en_US.ISO8859-1/htdocs/where.xml