Certification requirements..
From what I can recall, all transport category A/C are required to withstand a vertical load of 600 feet/minute at maximum landing weight and 360 feet/minute at maximum take-off weight, so there is a lot of margin to cover firm/hard landings, I know of one B777 operator with a 40 kt. crosswind limit and the normal procedure is to land crab on.....the torsional loads on the main gear attachment points must be incredible but the airframe obviously can take it, even at Max LDG Wt. of about 251,000 kg......