sixish
Doctors don't go around saying just "don't be harmful" or that as long as they have good intentions some harm is okay.
The simple fact is that formulation "don't be evil" is intended to avoid accountability. You can overlook some evil because you perceive them as having good intentions. Maybe you should consider that an advertising company may have put more thought into their motto and how it affects your perception of them than you realize.
A "do no evil" pledge is something that can be judged. Did they do evil by sabotaging the local 'yellow pages' in Africa? Was using their position on the board of competitors for their own advantage evil? This phrase refers to events and actions and facts, things that accountability is based on. Something can even be an honest mistake that does evil.
But "don't be evil" is an accountability dodge. Instead of judging actions and events it says to look at the company as a whole. Is an evil act threatening OEMs with no more Google apps unless they tow the line ok because it's all to 'unify Android'? Is it okay to sabotage privacy because otherwise Facebook will eat their lunch? Can two wrongs make a right?
The question of whether Google is evil is much more complicated and ambiguous than whether Google does or has done evil. That they insisted on not being judged based on their actions speaks volumes.