Why don’t these merchant-sailors have their own guns? Why do they have to spend money hiring mercenaries?
They are merchant marines, not military. Their training is for their nautical and technical skills - and they need to keep doing their jobs for the several days through the pirate areas, rather than trying to pull 24 hour sentry duty. Also, many ports don’t allow them to have guns, even simple rifles or pistols.
They seldom have real danger, as the ports are usually protected (by the governments, or local mafia). Where there are concerns (like the Horn of Africa or Straits of Malacca), those concerns usually effect enough ships, that local security services can make a good living serving as escorts through those waters.
What they typically do, is pick up security teams coming through the Suez Canal or Red Sea, or around Salalah Oman coming from the other direction. There is a range of options, from high-priced Israeli, American or British former commandos, to low cost third world roustabouts.
Also, ships typically bunch together in groups like convoys, and stay in touch with the anti-piracy Naval Task Forces in the area. The Egyptian Government makes commercial ships convoy through the Suez.
Interestingly in the movie "Captain Phillips" his ship is all by itself and not with the pack. Any idea why they did that? I would think that would be a big mistake in those waters.
Maritime laws against firearms on board were written for fear of mutiny.
Those mercenaries need to bump up to at least .30 cal.