What I said about Tim OReilly accepting money from the SEOs.
(In the comments)
+1 for the negative externality comment above.
Also, isn't accepting search engine optimization ads the same as accepting money for flyposting? Essentially, you're taking money for pasting adverts on someone else's (Google's) property. (That's why your "mere arbitrage" defence doesn't work.)
I'm surprised by the number of people who think this is Google's problem and responsibility to fix. It's not normally an argument in defence of flyposting that the problem belongs to the person who's fence gets flyposted.