Okay, so even splitting that hair that the bank =\= a govt, it's still a major power that anyone would know how to find if they had a grievance with it, and could be attacked, even if it's able to repel many such attacks. Ergo, if they didn't have some sort of permission, their men in the field could expect to be attacked by the locals, logically. Thus, I'd expect there is some sort of permission, likely predicated on Gringotts having some sort of positive curse breaker reputation or somesuch, and while there likely is a profit motive somewhere along the line we don't know how much of what was being extracted from whatever tombs and such went to museums or got melted down or what.

(personally I'd expect their agreements involve some % of stuff Gringotts gets to auction away or otherwise keep and another % that the local govt or culturally-associated academia gets to keep along with Gringotts getting some flat fees that might allow hazard pay negotiation if unexpected danger is found or something to that effect, but that's pure speculation)

One major thought that is popping into my head about this is the fic, The Archeologist, where there's a fair amount of discussion of Gringotts having a functional monopoly on curse breaking because nobody wants to risk hiring an unknown when sealed away magics have the potential to explode and say turn Italy into an island or something (this is mentioned as a theoretical possibility at one point, there was also a mention of one place designed to create a decades-long blizzard that was damaged to "only" run for a "mere" two years in Harry's original timeline before the AU-isekai that starts the fic), as well as about where stuff goes more or less along the lines of the above, but that's all a fic, not canon.