It's a surprise for me, because this is the first ereader capable device I've seen with such a limitation out of the box. The various Kindles (including the Fire), Nooks, Sony lineup, iOS devices, and even my WebOS tablet lets users sideload books. None of the reviews mentioned the issue, so it initially seemed like I was missing something when the Nexus 7 with the bundled Google Play eBook Reader didn't let me load my own content.

Also a bit surprising since the Google Play book store sells books that works on most of the devices in the above list. A friend of mine frequently gets new books for his Nook from Google Play.