I’m not sure it really showed this. All it describes to me is that any expeditioners ought to follow the rules so as not to piss of the Nepalese government. The U.S. gov’t did not know if such a creature existed so they simply covered their bases.
UPDATED (26-Feb-2013) Slate reports on this in their Vault
These regulations were actually first issued by the government of Nepal in 1957. The U.S. established diplomatic relations with Nepal in 1947, and the embassy had just opened in 1959, when the memo was written. The Yeti presence in a State Department document doesn’t prove that the U.S. believed in the Snowmen. Rather, by reprinting the Nepalese government’s regulations, the embassy could show Nepal that the U.S. respected its sovereignty, even in the matter of hypothetical hairy beasts.