I listened to the Dawes album on Spotify yesterday, it's very good... I think you need to like Jackson Browne though!
I'd never heard of them but I've been spending a bit of time streamlining my Amazon account so that the recommendations make a bit more sense and this was near the top of the recommended list.

I'm pretty sure that I'm not on a specific list but maybe the "better" your profile is, the more likely you are to get this type of email, after all, I wouldn't want every email that Amazon must send out (considering how targeted their emails are, there must be hundreds a week). As I said above, I spent a while improving my profile so that they don't think that, for example, just because I once bought a Coldplay album as a present, I might ever want to hear them again!

This certainly isn't the first Amazon free sampler - they did a Domino records one a few months back and, of course, they had a few pages of single track free downloads years before they actually sold MP3s.