Pinch on Newsstand doesn't work.

Newsstand is not really considered an app and thus doesn't act like an app would...it's merely a "glorified" folder. As an example, open Newsstand, then double-tap the Home button. Open apps are displayed here. Notice that Newsstand is NOT. It won't act like an app or have app properties, such as pinch to close, etc. To close Newsstand, simply hit the Home button once.

Exactly. I just edited my post because I neglected to include that. 😉

All that you have to do to close Newsstand is to tap the home button once. As you already know, you cannot pinch to close it or swipe up on the preview window to close it. As SergZak explained, it is not an app, it is a folder on steroids.