How? If the hammer is at full cock, it's protected by the grip safety, and since the 1911 has an inertial firing pin, a blow to an uncocked hammer isn't going to cause any problems, either.
Carrying a loaded gun at half-cock could be an issue, as the hammer would then be exposed to a blow, but "everyone" knows that half-cock is not a safety, and the gun should not be carried that way with a round in the chamber.
On a gun with a beavertail grip safety, you're correct. However, if the gun has the older style grip safety and spur hammer, then the force of being dropped could be applied directly to the hammer depending on the angle at which the gun hits the surface it's dropped against. Also, not everyone knows or belives that the half-cock notch should not be used as a safety and I've seen several people carry 1911's that way.
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