Why do sharks swim with their fins exposed?

Video footage of a great white shark off the coast of St. Andrews, New Brunswick, has raised a few eyebrows. Great whites are endangered in Canadian waters and rarely seen, although this is by no means a first. But I started to wonder why sharks swim with their dorsal fin exposed anyway? What advantage does this give the shark, or do they just like freaking people out?
My first attempt at finding out assured me that, nah, skarks don't actually do that, it's just something we have come to expect after watching poorly-researched Hollywood movies: "Sharks do not typically swim at the surface of the water with their fins
exposed...They are not doing it on purpose."
My second source? "Sharks in the wild very, very rarely swim with their dorsal fins exposed above the water...most sharks will not approach the surface at all". Third, and fourth sources (the latter specifically talking about great white sharks)? The same.
Well, the shark in that video certainly seemed quite confortable exposing him or her self for quite some time. So, what gives?
My fifth information source did finally confirm that, "surface finning is common behavior in several shark species...white sharks routinely investigate objects on the suface...following kayakers is not uncommon for this species or other large sharks".
Should I believe this source over and above the others? Well, just the fact that it gels with what we actually see here suggests to me that it may well be more reliable. Slate is a pretty reliable source, isn't it? So, what sharks that exhibit this kind of behaviour are actually doing is just checking out what's happening on the surface, just being inquisitive and nosey basically. The same article does point out that sharks cruising along the surface are very unlikely to be hunting; they are much more likely to hunt in mid-ocean or in the depths, and they would typically strike from below anyway.