Preparing a halfway decent digital image requires a great deal of skill, in the same way that a decent transcription of a written text requires a great deal of skill.
Someone transcribes a text of a manuscript, and according to Bridgeman, anyone can reproduce that text that the original transcriber established. This is precisely the same free-rider problem that legal tool of copyright attempts to solve. Of course copyright doesn't apply for a transcription because a transcription isn't a creative work, but there is still the free-rider issue and no legal tool available to solve it. For services there are trademarks, for processes there are patents, computer code is copyrightable or patentable, but for text there really is a legal hole.
Back then, before the digital age, image libraries would charge for reproducible copy, but nowadays there are lossless copies, and Bridgeman is very much out of touch. The German court, thankfully, had some better sense.