The Reference Desk/The Silence's death?

If in 1969, The Doctor placed a post-hypnotic suggestion into the moon landing that Silents should be shot on sight, how could they exist later in time (the 51st century?) when they abducted River Song, forced her into an astronaut's suit and had her kill The Doctor? The recap for The Day of the Moon implies that they were all killed after people watched the moon landing.