It's also probably a pretty good marketing tool for himself. More people play the module (especially online) because they have the maps to use... more of them then decide to actually buy the Curse of Strahd book in order to continue their adventuring... and then he makes the rest of the maps in *that* book available for purchase.

So he makes himself look good and possibly opens up more sales opportunities down the line. It's a good plan. Give 'em a taste, and hope they come back to buy the full meal.