Going back to real world examples, it's worth noting that construction methods for a combatant-type vessel (read, a warship) and a civilian vessel are incredibly different, but also share many similarities.
The best way I can give a general comparison is that your average naval frigate sailing the ocean today is around 300ft-400ft long and displaces around 3000-6000 tons. Your average container/cargo/tanker/etc. is twice as long and displaces four times as much. One frigate could sink almost an infinite number of container ships (limited only by ammunition) with literally no concern about reprisal whatsoever.
For starters, the merchant ships are totally unarmed, but they are also simply designed differently. Bulk storage, not survivability, is the main concern. Ramming would be their only method of fighting (which they would win, theoretically) except that the frigate probably has four times as much horsepower per shaft, is significantly more maneuverable, and has a reinforced internal structure. Historically, collisions between military and civilian vessels have ended worse for the civilian ship, though that's in no small part due to the MUCH better mobility on the part of the military vessels.
Much the same is probably true in Star Wars. Obviously any space ship needs to be vacuum sealed and resistant to the rigors and hazards of space travel, but if you take a large bulk freighter (which I assume exist though we never really see any) and change it's cargo bays into gun ports, it'll still probably get wrecked by the first patrol corvette to roll up on it blasting, simply because one is designed for warfare and one is not.
Ships like the Millennium Falcon are a bit different, since they're heavily modified. I imagine it's not laser cannons themselves which are illegal, it's the fire control suite and military-grade shields on the Falcon that make it so illicit.
Man, I should start a blog.