Perhaps checkbox could be added to not send location data, but that is a bit dumb I think, because ordinary user has no easy way to check if the checkbox even works etc. etc. nearly equivalent trust is still needed.
Everything you say is true, but a simple checkbox of "contribute to global public map (Note: this will expose your current location but will not be displayed publically)" alongside options to set your own endpoint server and mapping options would be absolutely tolerated by most people and not cause this sort of distaste.
You could even just not send current location along with the map tiles in which case 90% of the argument against what you do is moot.
The main issue is not that you have to be trusted anyway, the main issue is that you default to doing something, that can easily be seen as nefarious, completely silently that the player has no knowledge of, this literally makes the code untrustworthy. Regular players may not know about it and that's precisely the reason it's untrustworthy, it opens questions about what else is going on.
Edit: I actually really like your idea of using this to build a global map, but I think it needs to be managed better.
