First, our authors own their work, so they can sell their books wherever they want, including Amazon, iBooks or Google Play.
However, we don't directly sell Leanpub books on Amazon, the iBookstore or Google Play, for three reasons:
Many Leanpub books are published in-progress, and all their readers get free updates automatically. This would be really cumbersome for us or the author to do using third party stores. Also, an in-progress book is a community, and the best community is not fragmented across a number of stores, but is in one place.
We have a generous 45-day refund policy. Since we run our own storefront, we can have terms like this which we control, as opposed to being at the whims of a third-party store.