Lowering the "front page" $/mile rate could make the service immediately less attractive to workers, who can select between multiple services based on the apps on their phone. Lowering mileage paid is much harder for workers to detect, since it requires carefully checking each trip and measuring what's actually paid.
This could also be an "unintentional-intentional" bug, meaning its presence was just an accident -- but that the company has chosen to prioritize its engineering resources addressing issues that improve its revenue, rather than decreasing it.