The weight of a cubic foot of water is 62.4 pounds. If you have a fin keel that is approximately 6" wide by 3 feet long by 4 feet deep, that displaces 374.4 lbs of water. Hauling your boat out and letting the keel hang, would be the same, in this case, as adding 374.4 pounds to the middle of your boat.
-Will (Dragonfly)
It is not about the absolute weight, it is about the area over which the weight is distributed.
Using my 15K lb boat as an example. The submerged area of the hull is around 350 to 400 sq feet. In the water, this works out to about 37.5 lbs per square foot. If the boat is placed on 4 pads with no weight supported by the keel, the weight on each pad is 3,750 lbs per square foot, 100 times the load.
Hulls are constructed with very thick keel stubs and much thinner upper sections. Again on my boat, the hull around the keel stub is over an inch thick, just below the waterline it is about half that. When the boat is on the hard with the weight on the keel, the weight of the boat in excess of the keel weight is supported by the thickest strongest part of the hull. So instead of having 15K lbs supported by 4 sq feet of ½" thick fiberglass, there is only 10K lbs supported by 1" thick fiberglass.
Even with the weight supported by the keel, the hull will deform as it sags over the keel and pads. When properly blocked, the deformation is small measured in fractions of inches, but it happens.