As a former airline employee I can assure you that an airline can refuse boarding to anyone. If an agent at the check-in counter interrupts the "rule/policy" as for example "law" then I can assure you that they can deny boarding. One of the reasons is that the airline is financially liable if they allow a passenger (s) to board/fly to a destination that they did not have the proper required documentation to enter. This can result in a rather hefty fine. With the E-ticket I think the airlines themselves are still trying to workout the kinks involved. This is why you will see more leniency regarding the E-ticket "requirement". It is a lot of paperwork to complete when you deny boarding to someone. Been there , done that.