Have to say you are right.
Hardly a bug. But it's confusing game-play choice (in fact, just a left-over from old wall towers being able to shoot). I have a pending patch were I replace garrison for turret points on wall towers.
I don't know why civilizations became so standardized.
Before, everything was more genuine; now there's an obsession with making all the gameplay generic and standard to create an unrealistic balance.
OK! Something I'd like to add, because maybe there's a case (or plan), to have structures that can be both garrisoned and posted (this should be the term, instead of "turreted"), the Occupy Turret button could be renamed to Post (in contraposition of Garrison, for external vs internal discrimination, and ignoring for now a more radical possibility of rearranging units from the structure itself). Capture could still be achieved with the Garrison and/or Post buttons on enemy structures, and if in the future (or some mod) units will be able to capture fortifications by storming gates or ramparts, this distinction would already be supported.