Google has made a couple of changes to the rules surrounding incentivization of user actions in the Play Store, especially installations, meant to crack down on ratings manipulation and fraudulent installs. In short, Google will monitor any app that engages in incentivization for installs, and will take appropriate action, such as removing an app from the top charts, negating some install numbers, or removing an app from the Play Store, if they find that the incentivization is meant solely to inflate install numbers. Rules about incentivizing other user actions, such as reviews, are fairly similar; fraudulent reviews will be filtered out, and their ratings boost to an app negated.
To be clear, Google put in its blog post about the issue that it understands that some developers legitimately use incentivization of installs to grow their user base. Google recommends strongly against this practice, saying that incentivized installers are far more likely to leave the app behind than those found through organic channels, and are less likely to purchase things or otherwise engage with the app in meaningful ways. By Google's definition, incentivization is offering users money, goods, or any equivalent, such as in-game items that would normally be bought with real money, in return for taking some action that increases engagement with the app or makes it look better. Google does not go into detail on how it will determine the difference between incentivized installs and reviews meant to inflate an app's standing, and those that are meant to increase app engagement or grow the user base.
The Play Store's rules for developers state in no uncertain terms that they are not allowed to try to influence an app's standing in the Play Store through any means aside from organic marketing and making users happy. Incentivized actions, such as giving users in-game coins for running through the tutorial or asking users of another app to install your app in return for in-game coins, can help apps to grow their userbase and spur engagement for users who would otherwise churn, but the behavior's potential for malicious use is just as great. With this rule change, it seems that Google is continuing its recent trend of fighting ratings fraud head-on.