graeme_p - 7:03 am on Mar 2, 2013 (gmt 0)
In other areas of copyright use of snippets is generally fair use/fair dealing or similar. That is what Google has been relying on in the rest of the world.
Also, in this case, manually asking permission from each indexed site would be impossible, changing from permission by default to no permission by default is impractical, and changing permissions is very easy (a two line robots.txt will do it).
Google preview is pushing things, and I am surprised that it has not been successfully challenged. It is a lot more than a snippet. The same applies to large image previews.