The zero marginal cost makes fixed prices even more important.
You production cost per unit is basically the cost of creating the book. The more units you sell, the lower the production cost per unit. So bestsellers make bank and are profitable even when sold at extremely low prices.
In comparison a book with a limited niche will need to sell at a way higher price to be profitable.
People will only buy a fixed amount of books. The idea is to give books of niche audiences a fighting chance.
> Conceivably the combined works of JK Rowling or Tolkien could be sold as a single unit.
Yes there is always pitfalls and loopholes to watch out for when defining such laws. I don't really see how this adds much to the discussion, as something like that would clearly violate the spirit and intention of the law. If it is legally possible it is because the law is implemented badly. Which could be but is another discussion.