Not a memo as such, but BD are sensitive about carry on for a very understandable reason. A BD Boeing 737-400 crashed in 1989 in the UK killing quite a few people. The accident report indicates that a number of the passengers might well have survived had they not been hit by falling cabin baggage. The full report can be downloaded from the UK Air Accident Investigation Branch if you're interested:

http://www.aaib.gov.uk/publications/..._90_502831.cfm

If there is one thing that scares me about flying to this day, it's the thought that I have 50-100lbs of luggage hanging directly above my head, and all so people can save a few minutes wait when they arrive at their destination. I know that the lockers are safety rated but no one ever weighs the bags before putting them in the locker. I know that I can get my carry on size bag up to 40lbs without a problem, and a few of those in a locker would take it over the safety limit.

These things are also not just about safety IMHO. There is also the issue of passenger comfort on board. If an airline wants to have stricter carry on rules than allowed by the authorities or the ratings of the lockers, then they should be free to do so. To take an easy example, the A380 is rated for 850 passengers but most airlines are ordering them with configurations limited to 500-550 passengers.