Interesting discussion. And to be totally honest, I am one of those people taking pictures near or behind the hired photographer, but I'm doing so as a family member or as an invited guest. And if I'm shadowing the hired photographer, like at a recent wedding, I often end up helping the photographer (holding an umbrella in the rain for the photographer while her assistant is freed up for other stuff or if the assistant needs to go pose or position the couple), and I've also helped the photographer by bringing stuff to their attention (the photog. completely missed the symbolic decorations on the dinner table that symbolized what the couple believed in and modeled their life at home around, until I brought it to the photographer's attention). But, and take note, I'm not pushy, and I'm not hovering in the photographers ear nor getting in their hair nor in their way nor do I try to take over the photo shoot. But I do quite often get photographs that the hired photographer missed out on. And if I do end up shadowing the photographer I make sure to get a photograph of the photographer standing with the couple, and also take several of the photographer at work with the couple in the background. I've never been told to back off, never been told to stop, and have never had a hired photographer give me the stare-down. And just for the record, I don't intrude and don't offer help to every photographer I see working at an event (but I might ask about their equipment if I see them taking a break). And I always make sure to turn off my flash if it will be distracting.
As to the shot-stealing, well, as SSIM noted if the photographer wants exclusive ownership of the staging and setup then the photographer needs to find a place or time with limited access. And to take this out of the realm of family-wedding events, would it be considered "shot-stealing" if I'm at a park someplace and see a landscape photographer set up for a shot and I set up and take a shot as well? (Not sitting in the photographer's pocket, but not waiting until they are done either if I also happen to be there at the same time. Giving them fair space to not encroach into their comfort zone, but fair is fair and they don't own the scene.)
As to controlling image ownership, the same things are happening with written words. How can I personally control ownership of the words I just typed in, and how could I reasonably enforce copyright protection if someone "steals" my words and uses them as their own plagerized words in some context that they are getting paid for. It is not a lost cause, but sometimes the field mice do take over the barn for awhile.
Some of you probably hate me now. No problem, I'll be out in the barn taking family photos of the field mice if you need me.
'scuse me while I duck and run.