Companies pay some employees more for pretty good reasons. I don't think they're thinking "Jim's a dude so we should pay him $50K more than Sally." No, they're paying somebody more because they don't want them to leave. And if they are paying you less than somebody else, it's probably partially because they don't think you are as much of a flight risk. And if Google is wrong about these decisions, I'd guess we'd see…
Unfortunately for Google, paying women less because "They can get away with it" (ie, women are less of a flight risk), is still illegal.
What amazes me is the confidence with which people equate the work of engineers when they actually have no stake in making that judgement correctly.
You see, if you are paying the money and living with the results, you have a much greater incentive to make this call correctly.
And those who are unhappy with these judgements have a much greater incentive to equate engineers who aren't actually comparable. It's called politics.