Google occasionally updates its Play Store rules to weed out inconsistent or shady behavior in Android apps, and we're witnessing one of its larger clean-ups today. The company's new guidelines more explicitly ban device interference: titles in the Play Store can't modify settings or other apps without permission, and they can't install bookmarks or icons that pitch a third-party service. Google is also adamant that any in-game purchases of virtual goods must go through its billing system. As usual, new apps have to follow these guidelines right away, while developers with existing apps have a 30-day grace period to make any changes. The policies won't necessarily stop rogue code from sneaking into the Play Store, but Google can at least say that it gave fair warning.