Some advertisers have been pushing this grey line by using shady tactics in order to get app installs for some time now. These pay-per-install ad campaigns use the same scarevertising messaging we see from malware authors like; You are infected or System Alert. These alerts often redirect to legitimate apps residing in Googles Play Store, like battery saving and security type apps. The ads dont stick around long and Ad Networks have a difficult time preventing because of their small footprint compared to all the good ad traffic.”]
Source: https://blog.malwarebytes.com/cybercrime/2015/07/the-grey-side-of-mobile-advertising/