Lack of history. I worked there and this is correct. The motto originates in the very earliest days of Google when they - like most other software firms - feared Microsoft more than any other company. In particular they feared being locked out of Windows or discouraged via IE defaults, killed the same way Netscape was. This was a big part of the reason for the original Google Toolbar. It was at the height of the Microsoft trial, Gates had been doing a lot of questionable things and there was a general agreement that the company was Bad News. Slashdot's Borg logo dates from this time.
Don't be Evil was basically a kind of jokey collective agreement that Google wouldn't become like Microsoft. Of course twenty years later the industry had grown enormously, Microsoft is no longer the big bad wolf, many working people in tech have no recollection of any of this and in America, exist in an ideological pressure cooker that tells them anything that isn't left wing is "evil". So the motto had been lifted out of its very 90s and software specific context to become a stick for perpetual interns to beat people of the wrong political persuasion.
If this stupidity actually goes anywhere near a serious court case it wouldn't surprise me if this history gets dug up.
I interpreted it more generically as "make the world a better place... by organizing the world's information, and making it easily accessible".