On my Nexus 7, limited to what types of media I have, and hence can see buttons for: In Magazines, BUY and SUBSCRIBE are in caps. In Movies, RENT, BUY, DOWNLOAD, and PLAY are in caps. In Apps, INSTALL is in caps, however Uninstall and Open are not in caps. In Music, LISTEN is in caps. In Books, READ is in caps. It seems like for buttons in the top section, non-capitalized text is a fluke.
I was theorizing Cancel and Uninstall might be lower case because they're removals, but Open being lower case in Apps broke the theory. I'm tempted to buy an app just to check "refund".