quote from Wizofoz: "Overweight landings do often require maintenance action as for the same VERTICAL acceleration, there is an increase in the amount of KE absorbed by the structure (KE= 1/2Mv^2. Momentum = M*V) HORIZONTAL velocity makes little difference to the strains on the airframe. "
Correct, that's the only reason why jettison systems are needed. I often do inspections on overweight and hard landings. Both require to check the gears and visual damage. If found, you're in big trouble because a phase 2 inspection has to be performed, which can include disassembling whole gears, struts and flap to wing connections. Landings are more stressfull on a gear than takeoffs, let alone hard ones.
JAR 25.1001 Subpart A FAR 25.1001 Subpart E explains that a jettison system is needed if MTOW is higher than max go-around weight.
The difference in max takeoff weight in this is that they include higher flap settings, thus less climb performance.