Google's foreign-specific search indices allow users to find web pages from the overall internet as well as being able to filter content down to pages specific to their own countries and/or languages.
The problem is, there are quite a few overseas (non-US) users who use Google without clicking the button to restrict content to pages specific to their countries, and there are companies who desire to restrict their content from appearing to those users in other countries, either because they are not set up to sell to those users, or because they may be legally restricted from conducting trade with those countries.
I suggest that Google might want to allow webmasters to specify whether their content should be presented to users in their various country-specific search engines, for these reasons.
Google could offer this through an opt-in/opt-out interface in Webmaster Tools, or this could be pushed as another type of restriction which could be added to robots.txt files. Considering how many countries there are, it would probably be most desirable to add this as an interface within Webmaster Tools. Webmasters could go in and click a button for each country-specific version of Google that's out there.