I suspect what's happening is that the sites on which you're seeing this have a large farm of servers for displaying their merchandise and a few servers with SSL certificates for secure transactions. SSL requires a small financial cost for the certificates itself plus higher maintenance requirements and per-session system demands, so some companies want to push traffic that doesn't need to be secured off to the non-SSL web farm.
I moved my personal email off yahoo years ago because they refused for years to maintain ssl connections for email, once you authenticated to the SSL server the connection dropped back down to plain-text communication for all your actual email traffic. I think they eventually fixed that, but way too late for me.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill