http://www.techradar.com/news/google-has-finally-banned-lock-screen-ads-on-android

Apps, especially free ones, understandably sometimes have adverts, but while they tend to only be displayed inside the app itself, some are more intrusive, leaking out on to your lock screen. Well, not any more, because Google has just banned that practice.

In its new ‘Lockscreen Monetization’ policy Google says that: “Unless the exclusive purpose of the app is that of a lockscreen, apps may not introduce ads or features that monetize the locked display of a device.”