Sensor Dust spotting is the only thing that is allowed and not considered unethical. I believe that since the dust was infront of the sensor its removal doesn't constitute "removal of elements" from the image as this dust was not infront of the lens. Anything which was infront of the lens and gets captured in the image is part of the image and shouldn't be removed.

Unfortunately there are some other people (not you) who do a lot of cloning and don't tell us thinking that people won't know. However, anyone with a little bit of processing knowledge and who examines the image seriously will definitely know that someone has cheated. When people resort to cloning it actually stops them from developing their craft.

To stop dust spots appearing in the sensor, one needs to be careful while changing lenses. Always stop the camera and then change lenses. It is better to point down the camera when changing lenses. Despite all precautions at times dust spots appear. My Canon Cinema C300 camera for filming had two dust spots and I had to sent it to Canon to get it cleaned.