Oddly enough, this idea was exactly what WotC originally wanted to do, until they realized that they could think of no feasible way to accomplish it. You've called them out on an idea they've openly stated they have no idea how to do right.
Arguments such as this, saying that WotC would salve the wounds of piracy if only they did X, Y, and Z often neglect to acknowledge that accomplishing X, Y, and Z might be presently impossible. I can propose that WotC would kill the pirate market dead if only they made their PDFs impossible to copy by illicit means... and I'd be completely right. I'd just be proposing something that no one knows how to do.
Linking the purchase of a physical object with the unlocking of a virtual add-on requires that the physical object have some kind of access code attached to it. This code has to be such that a ganker cannot simply copy it out of the book and then go online to collect. Anyone who can think of a way to feasibly do this when it is a _book_ that is the physical object, something that is intended to be opened and paged through by a prospective buyer, should quickly communicate their insight to WotC. And as a tip, shrinkwrapping your hardbounds is not a live option.