In anthropology and other fields of study, there's a phenomenon known as the observer effect.
Although a chemical or physical reaction is likely to go on as it normally would regardless of who's watching, that's not necessarily the case when you're studying cultural behavior in either humans or animals. It's been shown time and time again that the knowledge that someone is observing us has an effect on how we act.
And for the purposes of a lot of researchers, it doesn't matter if we hide what we were doing before we realized they were looking or play it up. They wanted to observe our natural behavior and the observer effect can make that essentially impossible in a lot of cases.
So while the methods of one research team studying gorillas may seem strange as we explore it, it helps to know that they went to the lengths they did to prevent this effect from taking place.