The coast there is part of a protected "Site of Special Scientific Interest" (SSSI) and the beaches in question are sufficiently far from normal access points that they are remarkably quiet. A condition of the developer being allowed to build houses was apparently that this didn't improve access to the coastline - hence the fence. You can see it here:
I would guess that at least a few of the home buyers fully intended to "alter the deal (pray I do not alter it further)" after the developer sold off the last parcel and exited.
I'd expect there to be conditions in the sale that would enforce this - also the landowner in this case would be appear to be the business that runs the surrounding golf courses and hotel, so they're probably not going anywhere:
I would also expect the relevant government agency (possibly Scottish National Heritage) to take a rather dim view of anyone breaking the conditions of planning permission.
Edit: I'd expect the people who buy houses directly from the developer to be told about this. However, I can imagine years from now someone buying one of the houses overlooking the beach (NB they are lovely houses) and getting a nasty surprise when they ask about where the gate is in the fence.
Or they don't ask, and buy semi-permanent sea access for the bargain price of one angle grinder, some unassuming hinges, a latch, and a can of matching green paint. I'm assuming that most of the home-buyers would be Scots, after all.
The folks on the other side of the fence are certainly Scots with an interest in public access to Archerfield Woods from the coastal walking trail.
I very much doubt that such a fence would be patrolled or otherwise monitored well enough to prevent rogue gate installation from happening.
If this was in the U.S., the same people cutting holes in the fence would simultaneously be posting signs saying "no public beach access" and blaming the holes on outsiders bent on ruining both beach and neighborhood. The zoning and development board would quietly be enjoying their kickbacks from the developer, and winking every time the fence got mentioned.
They know that fence isn't going to last. It's only there so that no one starts a protest until after it's too late to be effective.
Actually, as far as I can tell one part of the planning agreement was for the developers to fund a ranger to help monitor the area - probably one of this lot:
So on further examination, It looks increasingly like this is an American-style gated community for the wealthy, and that the new home-owners probably wouldn't even know what an angle grinder is, much less which end was safe to hold. And on further examination of the fence, one pair of bolt cutters and enough time to make 20 unmolested snips would be sufficient to restore the previous state of the locals' Allemansratten (https://en.wikipedia.org/wiki/Freedom_to_roam).
http://www.geograph.org.uk/photo/102036
Edit: Keeping up my secret work for Visit Scotland - I can recommend the walk to the beaches from Dirleton:
http://www.walkhighlands.co.uk/lothian/dirleton-fidra.shtml