I for one like the idea of C-3PO shutting himself down voluntarily when he sees Anakin Skywalker's lightsaber. (See Inside the Worlds of the Star Wars Trilogy.) It does take a bit of creativity on Captain Antilles' part to put that programming in there (maybe Bail Organa provided more details offscreen), but it was also a safeguard against any blabbering from the protocol droid. We all know how chatty C-3PO can get...