Ads are just a fact of life in mobile apps. You can't completely avoid them, but there are some ad implementations that are so annoying that Google has explicitly disallowed them from the Play Store. Remember Airpush? The current advertising scourge is ad-infused lock screens, which have shown up in previously safe apps like ES File Explorer, Peel, and Hotspot Shield VPN. Google has finally listened to our pleading, and lock screen ads are no longer allowed in the Play Store.
Technically, the new policy is a bit more nuanced than "no ads on the lock screen." Here's the new section on Google's developer monetization page.
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.
So, an app that bills itself as a photo editor, VPN, or file explorer cannot also cram a new lock screen on your device that's infested with ads. However, an app that is actually a lock screen can still monetize with ads. Presumably, you know what you're getting when you install a lock screen app.
Source: http://www.androidpolice.com/2017/11/30/google-finally-bans-apps-include-shady-lock-screen-ads/ [androidpolice.com]