I don't see how this follows, unless we also agree that humans who have ever read any GPL code are themselves permanently tainted and therefore cannot produce anything that isn't influenced even slightly by said code.

Is it just because we think the robot does a better job at learning than we do? It's an impossible line to draw, I agree, but I don't agree that the answer is "well then everything must be considered tainted," I say the answer is "ignore a vestigial concern of a bygone era."