Because attackers wouldn't be able to send legitimate-looking data to GPS receivers any more.

Yes that's what spoofing is, but why wouldn't they be able to?

(EDIT: I see the other reply thread is already asking the same thing, didn't intend to ask about the same thing)

	▲	stavros an hour ago \| parent [-]
		Because, due to how cryptography works, nobody other than the entity holding the signing key (ie the one that deployed the satellites) can produce valid signatures for that key.