Sort of like when you donate $100 to NPR and they throw in a coffee mug, right. You're obviously not paying $100 to get a mug, it's just a nice bonus they throw in to thank you for your voluntary contribution. If all you wanted was a coffee mug, you could get that some other way without paying $100.
But this bundle isn't really a "bonus". $15 to get the best item in a bundle - the one that, to be honest, is the most exciting for many people - that's not really an "encouragement to donate", it's a sale. The deal is not "pay what you want", it's "pay $15".
Again, I'm not saying it's a terrible thing to offer that deal, but the innovative thing about the humble bundle was "pay what you want". The idea of empowering and respecting the consumer, rather than trying to manipulate them. Because we all know that ultimately, if someone can't (or won't) afford $15, they're going to skip the sale and grab the audiobook off piratebay. So why hold the information hostage at that price? Why not allow people to pay what they want for it, in a way that makes them feel good and generous, rather than like they're being forced to take a deal they don't like? This kind of bundle feels like it's using the trappings of gift economy and free information as bait, but then switching up to a conventional business model when it comes time to reel them in.