Google peers with many national and regional ISPs at exchange points. I don't know the exact numbers, but I'm guessing a fairly significant portion of their traffic does not go over traditional "tier 1" networks and rather is handed off directly to ISPs.
Putting traffic into a tier 1 network costs, taking it pays.
A massive surge from Googles hardware onto the tier 1 networks will be very, very expensive.
Last time it came up was as the BitTorrent protocol started blowing up ISPs that couldn't pay the bills.