when you've built up a vast printing empire, you kind of need to keep printing new books
Actually, it's not the printing of the books that keeps them in business, it's the contributions for the books. So, if they put the updates into an envelope with a tear-off contribution stub, or sold the updated pages at the literature counter, they would still get all their money, while reducing costs.
Basically, as long as they still get the money, they could forego publishing altogether. They're not really in the printing business. They're in the contribution business.