I don't see it as an obligation for the seller to offer a buyer a free electronic version of an book.
There are however publishers who offer either free or highly discounted e-books as an inducement to buy their print books. This is fairly common in with tech publishers like O'Reilly, No Starch, Apress, Informit, Manning etc. Several of them offer the opposite as well, buy an e-book and get the published book at a discount.
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