So having tons of AIs quoting various literary works and reproducing knock-offs of them has a positive effect on those books' sales?

I think you're wrong: there is absolutely damage to the authors and publishers from what the AI companies have done.

If you steal an unpopular product from a store, the damage is also only to "potential profits", so how does that differ? It's entirely possible no one would have purchased the product and it would have eventually been discarded/destroyed.

Or with services, if a barber cuts your hair and then you run away without paying them, do you not consider that theft, even though there's no change in ownership occurring?