Yes, it's disturbing how many sites still do this. Most of the big shopping sites like amazon.com, ebay, Target, WalMart are http until you log in. Was also until recently very common on credit card sites, though all of mine are all https now.
At least for most of those the login is a separate protected page rather than a https iframe.
There are still parts of the world where caching proxies are used to conserve limited transit bandwidth; CDNs don't solve that.
Only if you're taking about a browser cache. Any other cache couldn't intercept the data and cache it without alerting the browser (unless you mess around with installing SSL certs).