Let's say I blog about topics a lot of other people blog about. Let's say there was a Google Blogs. I would want Google to post a relevant snippet of my blog post to help me stand out from all the other links. And Google would happily comply, because this makes those who bought ads that now get displayed on my blog happy, and Google can charge them more money.
I'd say I would have to have a very twisted and self absorbed mindset to believe that Google is stealing from me for posting a relevant excerpt from my blog. But maybe that's just me.
This is true only insofar as it incentivizes people to come to Google in the first place. If Google can directly provide the information that the user is looking for, then it gains no benefit from sending the user elsewhere instead.
Second, in most cases those previews are too small to provide the information the user was looking for.
Your argument is based on false assumptions.
It's like saying that Google doesn't benefit from GMail because people reading mail doesn't make them money.
I guess an other way to answer this question would be to say: if Google doesn't care at al about previews, why did they develop the feature in the first place and then fought against these laws instead of just removing it as soon as people started complaining? Clearly they do deem them valuable enough to go through this whole charade.