The burden of proof that copyright infringement took/takes place is, juristically spoken, indeed on the copyright holder. However google adsense may see this in another way: It looks like copyright infringement or something inciting this, we cancel our agreement, you proof that we indeed did act wrongly on this. I did however also read that google adsense does indeed issue warnings, if it looks like copyright infringements. The user can then make this right. I think this is fair, as indeed many sites do have intereactive users (like forums, blogs, SB-sites, directories and the like).