I do not understand the Wikipedia definition of "scraper site".

By this definition webcache.googleusercontent.com qualifies.

It is a full copy of every site GoogleBot scrapes.

Google gives attrition to the original source, but if this isn't "scraping", what is?

They have been sued for this, and they've won. The benefits of a decent search engine outweigh the burden of infringing the copyrights of others. At least where Google and other search engines that cache websites are concerned.