Snippets are indeed a useful feature to have, but Google should share some of the ad revenue it generates with the publishers of the featured content snippets.
They do indeed generate revenue for the publishers, because a fraction of the users actually click through into the article. Without the snippet promoting it, the article would see much fewer traffic from Google.
The snippet is almost always generated from the first search result, which garners the lion’s share of the clicks. Now, many find what they were looking for in the snippet and forego the click.
The tricky thing about search AdWords, different from AdSense, is that for search AdWords, ad revenue comes in when the search results are bad.
Snippets generate revenue due to brand loyalty in the from of future searches.