Anyone able to view an image can share it with other people if they really want to - maybe they need to save it and host it themselves, or take a screenshot, or

Anyone able to view an image can share it with other people if they really want to - maybe they need to save it and host it themselves, or take a screenshot, or even use their phone to take a picture of the screen. Facebook has simply opted not to put any effort beyond the trivial into this.