So, Google has done this before. They do omething similar with dislikes on YouTube. They aren't doing it manually. When a bunch of pissed off assholes brigade an app and flip a bunch of five star reviews to one star reviews based on one policy change, that's generally not something that Google want to have too trong of an effect. If negative reviews continue steadily over time, they'll pile up.
Except Robinhood absolutely deserves a one-star rating. Sure, go ahead and remove the reviews if they're untrue, offensive or obscene, but negative reviews about an incident that actually happened shouldn't be removed.