i believe that is because in the the seiko world, there is this thing called modding. in the rolex world this would all be called fakeI would agree with you, somehow third party Seiko dials seem to be known as aftermarket but for example, Rolex dials are called fake.
Maybe the difference is that Rolex can supply 'service dials' which would be real Rolex dials but not correct for the watch, whereas Seiko cant so there is little option but to use a third party dial, semantics maybe but I see the difference.