Google, Facebook, Twitter, Amazon, and Stack Overflow, among many others, use form submits with full page reloads instead of AJAX on their login forms. I saw this question (When should I use AJAX to submit forms VS regular page submit) which says that AJAX should always be preferred if you can convey the various form states to the user, so I'm curious why these major websites seem to be going against this. Is there a UX principle behind this or is it most likely a technical reason?