It looks like you can use it for non-commercial purposes but I imagine that they'd make money a few ways: indirectly through publicity and familiarity (the more people are used to having it, the more might use Amazon's commercial version or just Amazon stuff in general), directly (through licensing it for commercial use), and via improvements they make to their service as a result of people using the free version for non-commercial applications (better voice training and interpretation).
That's just a guess but it's similar to what Google does with AOSP and Google Android.