The issue is that they did it INTENDING to profit which is ADVOCATING illegal activity.
From the way i understood it from the point of view of Google, it was an act of NEGLIGENCE on their part. Not exactly intentional, or advocating.
There's a lot of automated stuffs going on with Adwords, just like their Adsense. Some bad stuffs might escape the filters.
But Google has also been accused of turning a blind eye before. For example, the Google Ranking issue. They were accused of not punishing popular websites who are gaming their algos, in the premise that these sites were driving huge revenue to their coffers. They only started punishing when some journalists made a fuzz about it.
Just discussing it isn't a problem. Providing a step by step instruction kid might not be a problem (skating on thin ice?)... Telling people it's a good idea and giving step by step IS a problem.
I don't think Free Speech qualifies in the context of this Google issue. Because money changed hands. The central point here, is the ADVERTISING. And this was not free advertising or public service ads. "Selling" ads, is internet commerce. And internet commerce is regulated, and not free speech defined by First Amendment rights.
And free speech, is not exactly free if you got paid for doing it.
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Note that this was a settlement, not an open court case.
Possibly because Google had no intentions of challenging the facts presented to them.
Just like Michael Jackson's child molestation charges in the past, ended in a settlement.