I'd like to add to that. Currently there are 3 ports only which drops copper ingots. 2 Russian, and yes the Dutch one.
I totally agree with the idea that copper should be limited to some extend. But in truth if you're not Dutch or Russian you have absolutely no way to get Copper (I guess except maybe seal bottles loot RNG and AI trader cargo RNG?). I tried sailing there, there are player contracts for copper and even after Maintenance, the contracts are automatically fullfiled (i thought contracts for resources could only be filled by players and not automatically by the town resource drops). So as a player of another faction, that means there's no way to gather certain resources for some of the bests and significants upgrades of the game - even if he is willing to do the effort of long sailing time and put up with the dangers of being a trader in enemy water.
I just think that it starts adding up to the port bonus situation.