The entire in RP point of Docking Rings is to make landing and taking off economical, a fighter would probably use up a large portion of its fuel without it, and the costs would probably outweigh the value of the cargo for transport class ships.
Sure unlawfuls COULD land on planets and make basses on the planets, but why would they? Their ships would use up massive amounts of fuel when launching and satellite tracking would make it outrageously easy for lawfulls to find the bases and in turn simple flatten the base from orbit if its in a remote area, or take it with military ground troops, vehicles and fighters that are operating in the atmosphere. (The Valkyries in SP demonstrated how effective fighters are against land targets.)
Why go through all of the risks when they could easily set up a space station or take over an abandoned one and operate safely away from the eyes of the law with the big guns? Yes there are probably unlawful bases on planets but they probably fit into one of two category. A) They operate exclusively on the planet with little or no contact with the space forces or B) they are the "fences" for the assorted smugglers that sell the goods there.
