My view on this might differ from the majority here, and some may not be able relate to what I am trying to say in the manner I am trying to say it. This is just my opinion and admittedly a very philisophical one, but it's still just one opinion..
The owls in this case are mostly nocturnal hunters therefore the more energy they expend keeping their eye on photographers during the day, the less energy they'll have to hunt effectively at night. I have heard reports of owls that didn't seemed bothered by the presence of humans being found dead from malnutrition in the same location that crowds of photographers had been photographing them day after day. If an owl is focused on something other than it's young, it's mate, or it's prey, it's probably expending energy guarding against potential predators. What else do owls do? Although photographers may not actually attack an active nest, the owls must see us as a constant threat and therefore they'd always be on guard when we're around. The threat of us photographers might not be enough to flush the owl from the nest, but my uneducated guess would be, the threat is still very real to the owl and they'd constantly be on the alert and using precious energy to guard against us potential threats, maybe not having enough energy left to defend against the real predators or hunt effeciently later.
Don't get me wrong, I have been guilty of mobbing wildlife myself and my girlfriend has the pic of me getting within 15-20 feet of a roadside Algonquin moose for a photo to prove it. I admit to being a hypocrite but in hindsite I know that I was invading the space of a wild animal and the closer and more frequently I get to ANY wild animal, the less wild it will eventually become. I can't be a part of the wildnerness that I enjoy without affecting that wilderness just with my presence. I'd like to think that there is no negative impact by my presence around the wildlife I enjoy so much, but the fact is, I can't think of any positive influence my presence could have in ANY truely wild setting. The list of possible negative impacts my presence could affect on wild space is endless though. I am a hypcrite though and I will still try to be at one with the wildlife around me. I will do everything in my power to make sure that I don't have a negative impact on the well being of the wild creatures around me, but I am also aware that the best of intentions can sometimes have negative consequences. One thing that I firmly believe is this, the less human exposure wildlife has, the best chance it has of surviving as wildlife.
Having said all that, I think I will start planning my springtime camping trips to all the bird migration hotspots so that I can join the masses of photographers looking for that once in a lifetime National Geographic shot.