This is a good point. However, I think it should be downgraded to a warning, rather than being outright forbidden.
While locking yourself out of HTTPS is a terrible idea on its own, both Amazon CloudFront and CloudFlare (off the top of my head) offer SNI-based SSL certificates that can be put in front of a bucket, even if the default certificate would no longer cover it.
CloudFront can be configured to pull from an S3 bucket of your choice (even if it’s named something different), but also costs money. Well invested money, one may argue, but money nonetheless. If one does not need the additional functionality of a full-fledged CDN, CloudFront is a slightly overkill solution to the problem of “I want my forum to load faster”.
(An incredibly convoluted pricing model doesn’t help… I think there’s an XKCD for this, but I can’t find it.)
CloudFlare is not as configurable, but also offer quite a few other features as well - as well as the very approachable price tag of free. With a properly named bucket and a DNS rule (possibly supplemented by some caching rules), you get an instant improvement to loading performance, with no downsides whatsoever.
Perhaps not as big of a difference as a full CDN would make, but hey, it’s free performance.