Camps can be captured without fighting

This bug doesn’t happen all the time, but on all maps, it sometimes happens that you can capture a camp without fighting, and it spawns Blackheart’s coins right after, even if you don’t play on Blackheart’s Bay

Can you be more specific? How exactly is it being captured then? Do the mercs just die on their own?

The coin is just a visual indicator/icon after someone bribes a mercenary.
It’s not an actual coin you can pick up. It means that mercenary was paid to fight.

Read up on the Bribe feature on heroes that have it. It allows you to capture mercenaries without fighting.
For example you can read the Pixie Charm feature of Brightwing in the shop.