Access is limited for both inRP and OORP reasons, most of which are usually legitimate.
The OORP reason generally being that the people who spend time and effort on constructing a base, don't want to see it robbed of all resources, undoing all the work they put into the place.
The inRP reasons range from things like diplomacy to national security, you wouldn't see a Xeno docking on a Police base, and you wouldn't see somebody unaffiliated with a Military organization docking on a military station without prior clearances and all that jazz.