Keep in mind also that the nominal fee creates accountability that, in the end, protects you.
By charging a small fee, WotC makes it impossible to anonymously take ownership of the digital version of the book. Anyone who enters a code to get the digital version must identify themselves in a real, positive manner (through the use of a credit card or other verifiable payment scheme). That makes the fee one of (probably several) roadblocks to piracy.
[And before this blossoms into a discussion of intellectual property rights, remember this: What you're downloading is a unique digital object assigned to the book you bought. If somebody steals that, they've stolen your right to access the digital version of the book. YOU are the loser in a very real way, not some giant corporation is a very intangible way.]
[And before this blossoms into a discussion of how the pirates always find a way, remember this: This will not be perfect in preventing piracy, and neither will any of the other steps WotC takes. But, combined, they're intended to keep theft to a minimum. WotC has a lot of experience with digital objects (via Magic Online), so their overall system will probably be pretty effective.]
So the requirement to purchase increases the likelihood that when I buy a book, the code to obtain the digital version is likely to still be valid when I take the book home and type it into DDI. That alone seems to be worth an extra buck or two.