It's fun looking at unintended tack-on effects of decisions like this.
For example, requiring SSL for all assets served on SSL pages is going to make the profits of CloudFlare, and other CDN providers with their same business model, spike precipitously. You have to have a paying plan ($20/mo to start) to get SSL CDNing support, which basically means CloudFlare's free plan is now useless to anyone who enforces HSTS.