tripper1000:

 

From a CAA/FAA and aircraft design standpoint, it is technically allowable for passengers to bring >7kg into the cabin provided the locker weight limit is not exceeded. You will note that during the safety briefings you are told to stow stuff under the seat in front of you - the floor has a much higher loading limit than the overhead lockers so the American airlines will be achieving compliance through this strategy. 

 

British Airways technically allows each person 46kgs of hand-luggage, if you can fit it within the dimension restrictions.

https://www.britishairways.com/en-gb/information/baggage-essentials/hand-baggage-allowances