Doesn't the obvious, honest answer make the most sense? Someone in Google management doesn't want their products acting unethically - merely because it is unethical?
The department probably exists in the first place as a result of an internal political struggle and compromise - with some decision-makers wanting Google to be genuinely more ethical, and others wanting to avoid any genuine oversight.
As a result, you have this bodge solution where the department gets to exist (helped by the PR benefit as mentioned elsewhere), but not to act independently. Anyone in the department offering genuine oversight is at risk of being forced out.
I certainly hope you are right. And with some foresight, the ethics department is a "Golgafrinchan Ark Fleet Ship B" now fully loaded and ready for departure. Bon Voyage!
https://hitchhikers.fandom.com/wiki/Golgafrinchan_Ark_Fleet_...