alt Hacker NewsThat doesn't answer my questions although I now realize I made a bad assumption. This is a modern technique. I misread the bit about Greece on the wikipedia article and somehow got it in my head that it was more ancient.
So I think to answer my question more directly, yes, it's just copyright laws.
My point, before understanding that, was that from the map consumer's perspective it doesnt make a difference. And from the original creator's perspective it has a little accuracy cost. So without an additional mechanism I didn't see the disincentive for copiers. So I wondered if it was some ancient form of copyright law or just strong sense of morals or what.
Deliberate traps are a modern 20th-century practice. In old times, each mapmaker made their own innocent mistakes, and those mistakes got copied into other mapmakers' work. In other words, historians can plot "family trees" of maps, and figure out who copied from whom, based on which errors appear on which maps and on what date. Apparently California drawn as an island lasted about a century because engravers copied each other.
There were a few factors discouraging mapmakers from blatantly copying each other:
Rulers granted printing monopolies from the late 1400s. A mapmaker like Ortelius would print "cum privilegio" (lol) on a map. If you copied it, your expensive copper plates could be seized, along with a hefty fine.
Spain and Portugal each had a master chart which were state secrets because they held navigation knowledge worth a fortune. Portugal handed out severe punishments for leaking charts. (Leaks still happened: the Cantino planisphere was smuggled from Lisbon to Italy in 1502.)
Booksellers' guilds kept registers of who owned which work, and enforced it.
And lastly, copying meant hand-engraving a new copper plate, which took months of work and real skill. The practical barriers alone got rid of most of the copiers, but it didn't stop the big houses.
Some followup links for the curious: https://gist.github.com/shawwn/b85668a421aa443879b15aedd10c5...
(I used Claude to research this fascinating rabbit hole, so I'm basically paraphrasing here. But this comment is my own original writing, so treat it as you like.)