The only time you have to pinch and zoom in portrait mode is when you're using a phone or tablet with a crappy resolution.I'm guessing it's logical to assume people would want to view a 'page' in a portrait view but having to pinch and zoom seems much less desirable than viewing the whole width and scrolling to read.
Even on a smaller 4.3" HD smartphone where text is generally pretty small unzoomed landscape, I still think it's preferable. On a 7" plus tablet, why use portrait? This is pretty damn odd to me.
You don't ever have to do such a thing on the Nexus 4 or Nexus 7. I've never had to such a thing.