@Saladman
This thread has wandered over several topics. The original topic is what is legal. It changed to a discussion of what would actually be used. I made a specific assertion within that topic. Though I don't think it is limited to the core (see below on that score). It is certainly the case that it is inside the Imperium and does not apply to folks traveling the Reaches or whatever.
How things actually work in your campaign is going to depend on a lot of factors. You can certainly make space travel super dangerous and rife with piracy of the 'ships fighting' kind. You probably need to throw out the level of Imperial naval resources talked about in the Imperial Navy sourcebook and make assumptions that space stations with fighter patrols are not affordable by interstellar ports or the shipping megacorps. As I said, in that case, commercial freighters would look more like blockade runners. Or they would move in convoys.
Deterrence is certainly a relevant factor. But the way loss prevention works is that you find the techniques that have the highest net reduction in losses. If you can reduce losses by 750k at a cost of 200k or you can reduce losses by 1 million at a cost of 600k, the business is normally going to go with the first option. And that is going to depend on how things work in your campaign also. If you have murderous pirates that can steal entire ships and vanish to pirate havens where they can resell them and the Navy can't do jack about it, then that's a very different environment than one where the pirates are going to steal cargo or maybe ransom the ship at worst.
Is spending millions on weapons and additional crew on every ship actually the best way to deter piracy? Fighting will almost always result in damage to both ships, which is quite expensive. Is it better than running Q-ships? Sponsoring local patrols? Paying protection money? Losing some insured cargo from time to time? Going to a different place entirely?
If you assume piracy is life or death guns a-blazing, being armed is probably the way to go. But that's not very likely inside the Imperium. It is going to hijackings, false emergency scams, or some kind of disguised/stealth ship that lets the pirate get in the merchant's face before the merchant knows there's a problem. Because if the pirate's actual plan is a space fight, they are either stupid or they have a paramilitary vessel. Which has armor, combat programs, possibly military grade weapons like particle accelerators and other things that means your ship has to be heavily and expensively armed to fight it off.
The swarm of belters is going to be an ambush situation because if it wasn't, you'd accelerate away. And carrying turrets is not a defense against misjumps. If you manage to 1) misjump and survive 2) Arrive near where a pirate happens to be hanging out 3) Still have an functional ship and crew able to put up a fight, then I guess that would be a win for your weaponry expense. That's not going to be a result on the actuarial tables in accounting
The classic "Traveller Adventure" is set around Aramis/Spinward Marches, so I'll use that as an example. You have a trade route of: Rhylanor (A/Naval Base) > Porozlo (A) > Jae Tallona (A/Navy) > Celepina (A/Navy/Scout) > Henoz (A) > L'Eoul D'eiu (B/Navy) > Aramis (A/Navy) > Natoko (B/Navy) with spinoffs that include Vinorian, Nutema, Margesi, Saarinen (all A or B, most with Navy or Scout bases). And almost everything else around there is B or C. The places that aren't are mostly low pop, amber zones, isolated by empty space, or otherwise not where you'd send a cargo freighter. Those places are where you send tramps because the volume is too low to support a 1000 ton ship, much less an actual heavy freighter. (The PCs, being PCs, naturally take on jobs that take them off the standard route and into amber zones and other dangerous places).
Another popular region: Regina Subsector... almost all A, B, C starports and about a 30% Imperial space base chance, except Forboldn (corporate owned world with Imperial ministry of Colonization support), Knorbes, which is an imperial reserve and research station with naval patrols on station. There's Whanga, which is a practically empty system a Jump 1 vessel needs to pass through, and Dinom, which is corporate dominated system.
Yes, Free Traders are in a different boat from commercial freighters. They are going to be the ones going to the half arsed backwater planets, not the commercial freighters. They are going to be a lot more likely to try to skim fuel at a gas giant where pirates might hang out. They are going to be more likely to be smuggling or dealing with unsavory sorts voluntarily. And they aren't going to have the clout for other solutions. Though they'd try to rely on alertness and spotting danger in time to run away. Fighting would a distant third on the options list. Still, they'd carry weapons if they could afford them and had the crew to use them.