[seafarer]

Certainly not a bug. All countries at war tried to arm their merchant vessels, or at least as many of them as they could. Allied merchant ships were initially armed largely with surplus WWI guns (the Royal navy had a lot of 6pdrs, 12pdrs, 3", 4" and 4.5" rifles in warehouses).

In the game, perhaps the only real anomaly is how good the merchant gunners are. But their guns can very quickly inflict major damage - you want to try to keep as narrow a profile presented to them as possible (makes for a harder target to hit), and at least don't hold a steady course for more then a few minutes at a time.