Devices are meant to be landscape oriented because our field of vision is landscape oriented - and it feels natural to have most of your field of view occupied by what you're primarily engaged in.
This is the reason books are landscape oriented once opened, so are newspapers (broadsheets both when opened and folded, tabloids when unfolded)
Also our shoulders are about 1.5-2 feet apart - so portrait orientations are uncomfortable to hold.
Columnar rivers of text, when laid out correctly (without requiring a vertical scroll) on a landscape oriented tablet "feel" natural -- and is the right thing to do instead of filling empty spaces with attention stealing widgets.
Infact you can attempt similar multi-column layouts on the web, albeit with limited success, like on most of the apple.com site and the erstwhile International Herald Tribune iht.com site (before it got merged into the NYT site)