B3y0nd3r, this is an excellent topic you bring up. It has many flavors and nuances besides the specifics you spell out. For example:
1) What if you add this to your story: You're an out-of-towner, and he's a local guy ? So now not only was he "first", but you're also an out-of-towner (that perhaps, like in AARC's story, fears you'll "leave holes"). I mean, go figure: It is true that the more md'rs descend on a site, then the more sore-thumb status it attracts from passerbys. Who might ask "gee, what are all those guys doing ?" and "gee, is that allowed?". Scrutiny that might never happen with a single lone ignored hunter. So put yourself in the local long-timer's shoes.
BTW, I would never have the above attitude. I actually enjoy meeting others, co-hunting, sharing, etc... But just saying, when I've run into the psychology you spell out, there IS the "other person's side of the story".
2) What if 2 guys did the research together ? And if the site turns out to be good, can the co-researcher announce "don't tell anyone, and don't go back" ? What if he never gets his duff in gear to ever even hunt there again ? Doesn't your co-finder status allow you to hunt there on your own ? Or even invite a buddy ?
3) What if he merely casually mentions this site, in a long list of 6 or 8 different sites that the 2 of you are co-reconn'ing. And all he did was snap a few pictures on his smart phone to text you, and you , likewise, had been feeding him your reconn. results. And it just so happens that one of his reconn. pix spots was the beach that was "going off". And let's say that you'd only just met the fellow on-line, and .... after finding the eroded spot, you try like heck to get ahold of him. To no avail. He no longer answers his phone or texts or emails. So can you hunt it ? If it's large and un-ending, can you invite friends (before it sands in and ends).
In the above scenario #3, I can certainly see where the guy can "make the calls", but what if he's nowhere to be found, and never made any restrictions?
4) What if the person you run into there is your long-time old hunting partner. You show up at an eroded beach, at either a) the exact same time as him, or b) he was there first that morning. But in any event, you both reconn. the same beach and bump into each other. Un-known to him, you had made arrangements with a few other mutual friends of yours to reconn. multiple beaches, and share results (tit-for-tat, since you can't reconn. 10 beaches alone in a single day). But before you can get on your phone and share the agreed-upon terms with your other friends, your first friend comes up to you and says "don't tell anyone". Now you're in a pickle. Because this first guy is ALSO your friend, and you don't want to offend him.
Interesting topic !