At the end of ROTS,
Jimmy SmitsBail Organa tells the dayplayer Extra to have the protocol droid's memory erased.
I'm pretty sure we can assume it's because of everything that Threepio has seen and heard, mostly about the twins so that the secret of their existence and locations can remain.
But why not Artoo as well? Astromech droids can only communicate out loud with unintelligible (to humans) beeps, boops, and bops. But that doesnt' mean that R2 couldn't be a source of information, via a download or another English speaking protocol droid. Right? Artoo poses an even greater security risk because of his ability to plug directly into other computers.
So why wouldn't
Jimmy SmitsBail Organa also wipe Artoo's memory?