TL;DR someone apparently cloned ZXing Barcode Scanner, added annoying ads, uploaded it to the Play Store with the same name. Soon enough the malicious clone got taken down. Legitimately pissed off people who installed the malicious clone are leaving angry reviews for the non-malicious original (presumably because the malicious clone is gone from the Play Store).
TL;DR someone apparently cloned ZXing Barcode Scanner, added annoying ads, uploaded it to the Play Store with the same name. Soon enough the malicious clone got taken down. Legitimately pissed off people who installed the malicious clone are leaving angry reviews for the non-malicious original (presumably because the malicious clone is gone from the Play Store).