WotC isn't charging you anything for DDB, Curse is. Curse has to pay WotC whatever amount their licensing fee is, and that may be per sale or per some other metric, but it's not the purchase price. Curse sets the price to cover the amount of work they're doing establishing their database and coding their tools and providing server bandwidth PLUS the license fee. WotC doesn't actually ever sell you a book, either -- you've never bought a book from WotC. They sell to distributors, who sell to storefronts, who sell to you, and at each point is a markup. Amazon cuts out the middleman and deals in huge volumes so they can cut their profit margin to razor thinness, and that's the only place you pick up near $30 prices on books. Most places sell them at MSRP which is $50. Trying to complain than DDB is selling their forever access for the same discount as the biggest, most aggressive bookseller in the world is entirely misplaced -- either way you're getting $20 off the book price.
The insistence that the only cost for DDB is what WotC charges for the IP plus a tiny distribution fee is just weird given how amazingly wrong it is.
Here's the thing, you can get access to SRD material for free on DDB. Plus, for free, you can create and share homebrew material. You can make everything you could want yourself for free using DDB. If you want the PHB or other books already there for you (the cost of which Curse is using to subsidize the free access costs of development, maintenance, and hosting, and pay licensing fees -- all non-trivial costs you don't have to pay if you don't want to), you can buy them. Since you've already bought them, you can just add the material you want to your free version of DDB.
Imagine how fair 'free' is as a price.