Its an old rule, that was designed when seats reclined to a depth that would actually hinder evacuation of the pax in the row behind.
These modern planes dont have seats that recline far enough to hinder!
Any reasonable (international flying) airline's aircraft has sleeper seats in business and first, the majority of these do not recline into the space of the rows behind, so that isnt an issue.
It's an old rule, from the 'lounge-recliner' aircraft seats of the past, and should be scrapped. The overhead locker comment is not an issue - the 747's overhead lockers in economy cover almost 3 rows of seats, it would matter not the position of the seatback if those things managed to fall from the ceiling.
Again, yet another rule within this industry that started in the day of the dinosaurs, yet never kept up with the modern aircraft we fly today.
