Why not?
For two reasons.
1. They're not taking a loss on every ebook in their selection solely to drive up Kindle sales. Like a few people have stated previously, what they're doing is selling some popular titles below cost as a loss leader, something to get people in the store so they'll buy other stuff that's priced higher than wholesale. Amazon makes a profit off ebooks overall.
2. They're not selling so low that no one else can compete. If Amazon had a setup like "buy a Kindle now, get all ebooks in our store for 50 cents a pop for two years", that would be predatory pricing. It would likely move a ton of Amazon hardware out the door and drive the competition out of the e-book business pretty quickly. No one could match Amazon on price without cutting themselves off at the knees in the process.
What Amazon is doing is selling some highly popular books for a buck or two less than what they bought it for, and making up the difference elsewhere in the store. There's absolutely nothing illegal about that.