The people who advertise by with AdSense might not be doing
evil, and it is true that they would be hurt by a boycott.
However, boycotts always have "collateral damage" and I don't
think that any companies would be seriously damage if they can't
advertise on your web pages, or indeed even if they can't advertise
with AdSense at all.

The point is a different one. The point is that Google itself also
profits from the AdSense ads. The boycott would be aimed at Google,
not at the folks who advertise with AdSense.

The danger is that if Google is seen as the least of all evils, then
as long as this remains true, no matter who evil, in absolute terms,
they are, then they will become evil. A high-profile boycott
directed at the China problem might wake up some people.