Because now you have to serve every single bit of your page over SSL (to avoid security warnings) and that means none of your page content can be cached. It also makes relatively mundane things, like having your proxy server communicate the originating ip address, much harder. I can set up haproxy to add an X-Forwarded-For header in almost no time flat. In fact I just gave you enough information to google that solution for yourself. Solving that problem over SSL is much harder.
Engineering a MITM attack is much more technically difficult than snooping traffic. Not every company actually need to turn the security knob up to 11 on this aspect, and being able to do unencrypted-page-with-encrypted-login is a good trade-off when you can make it.
For what it's worth, if you want real HTTPS security, you can't serve pages from your "regular site" over HTTP -- especially pages with a login form on them -- doing so is an invitation for a man in the middle to step in and phish your visitors.
Yes, this is fundamentally insecure, and I am aware that this is not a problem with Backbone specifically, but a general problem with the approach of building browser apps without page refreshes over HTTP for everything except for the login screen.
