it all has to do with leverage.
those seat posts can carry a 200lbs rider no problem, but the weight is not sitting too far out on the 'arm'
with a rack, the 'arm' is extended very far off the body and has much more leverage to break the seatpost or body.
it's probably the rack, more specifically, the clamp, that's rated to 25lbs and takes into account hitting a curb or large pot hole which can put a lot of G-force on arms outside the body.