Amazon cares much less about your security than about the fact that tiny increases in latency decrease conversion dramatically.

This is because most people are browsing and not buying, so the caching advantages of unsecured catalogue pages is likely quite large.

At least for most of those the login is a separate protected page rather than a https iframe.

Uhh, what caching advantage are you talking about? Everything that can be served cached over HTTP can also be served cached over HTTPS. Unless you are talking about a caching proxy outside of the website control. In that case, they aren't needed, CDNs solved that problem in a much better way.

> Unless you are talking about a caching proxy outside of the website control. In that case, they aren't needed, CDNs solved that problem in a much better way.

There are still parts of the world where caching proxies are used to conserve limited transit bandwidth; CDNs don't solve that.

"Everything that can be served cached over HTTP can also be served cached over HTTPS. "

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).

I wonder if even simply the overhead of setting up and encrypting the secure connections even just by itself has a measurable effect on people's buying behavior.

Yes but the "login" link, the "add to cart" link, etc. is all served over http so could be replaced by a MITM.