Yes, and their names get splashed across the media. Thanks to the media, people know where Chapman lives (or at least the street he lives on) and his full name. He might've feared being threatened by whoever took William if he spoke out. He might've thought the police had it under control (in the early days, the media was heavily focused on a boy missing in the bush, not an abduction.)
I wonder if any of what Chapman allegedly saw ties into the other witness report of a large woman acting suspiciously near a 4WD on the same day. It was reported early in the case. I'd love to find the source but it's gone