The problem is you can't just start arming merchant ships and then expect a country to allow them into their trading ports-of-call...
"Don't mind that big 50mm cannon on the bow...it's just for protection. Honest."
Would we (the US) want some ship from the Mid-East (or anywhere else, for that matter) coming into one of our ports and docking there when it has big guns on it?
The peaceful merchant trade ship becomes a ship of battle when it gets armed
... I firmly believe that any man's finest hour...is that moment when he has to work his heart out in a good cause and he's exhausted on the field of battle - victorious.