It puzzles me as to why camera crews are not acknowledged in many different doentarys by per-by's ?

im quite amazed at how a TV crew can just doent a program even with an obviouse, well-known journalist, celebrity etc.....Anyway why dont they get people surrounding them for example...on the secret millionaire how they just walk into a rough, tough, british pub with a camera crew and yet they just sit there and carry on wiv their own business???? are they warned in these kind of situations that crews will arrive at some point ??????