The rule isn't just to prevent a solo pilot from going rogue. There's also the issue of medical emergencies. Cockpit doors were reinforced after 9/11, and that created a new risk of a solo pilot having a medical emergency and the other pilot not being able to get back into the cockpit to help. Considering that all US carriers manage to comply with this rule, and that it doesn't really cost anything to implement, it's a bit puzzling as to why it's not universally done.