Well, let me show you the difference:
1. If your ally captures a mine, it will remain inaccessible to you, and all the resources will go to their treasury. How much of it they will send to you is unknown, but it won't be very much, I agree...
2. If an opponent captures a mine, you only need to visit it to make it yours – and your ally won't be able to recolor it to their color...
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