Basically that Google is aware they are using robots in conjunction with AdSense and despite the fact that Google understands this is an art project, Google has to defend its AdSense program as well as its advertisers from harm. Based on the material on the web site itself, Google believes the GWEI people give the impression that their project is violating the AdSense TOS. Google will unequivocally defend its rights and this generally includes shutting down the AdSense accounts of violators. Google also reserves the right to take additional action.

I.e. not quite a takedown letter. More like "we're watching carefully".