It's an interesting move for Google, who's tools are almost all open and free to use. The community has gotten large enough where there is a market for paying to attend their event and at least cover their cost of employees answering (often innane) questions about how to get more web traffic and AdSense revenues. By charging a fee it also limits attendance to something reasonable.
The agenda also includes what they are calling "lightning 20 minute 'unconference' sessions" - where attendees "vote" on topics they want to hear presented by other attendees. I think that sentence could have included a couple more buzz-words.