Maybe the kid had previously complained that the littles ones, were really bad kids, and the parents were looking at their kids...
I think the parents would and should know if their kids were "really bad" to begin with and wouldn't need a video surveillance to find out. IMO, if a teen babysitter told me my kids were behaving badly in his presence, I would probably look into a different care taker. I wouldn't trust the teen babysitter would be able to "discipline" misbehaving children properly and I wouldn't assume he could and want to watch behind the scenes without being able to influence how everyone was behaving, especially someone watching my children first hand. But that's just me. You may have a point to some degree with these folks, as it seems they aren't too bright.