"The licenses Wikipedia uses grant free access to our content in the same sense that free software is licensed freely. Wikipedia content can be copied, modified, and redistributed if and only if the copied version is made available on the same terms to others and acknowledgment of the authors of the Wikipedia article used is included (a link back to the article is generally thought to satisfy the attribution requirement; see below for more details). Copied Wikipedia content will therefore remain free under an appropriate license and can continue to be used by anyone subject to certain restrictions, most of which aim to ensure that freedom."
Besides, Wikipedia isn't an ad-supported site, so Google's practice of including their text in the search result doesn't cost them ad revenue. You could argue that it means people will see the banner requesting donations less often, but that's compensated for by Google's own donations to them. So while I have definite concerns about some of Google's other practices, that one is no big deal.
From https://en.wikipedia.org/wiki/Wikipedia:Copyrights