This is basically a measure of which search engine promotes Wikipedia links more heavily in their algorithm, not a measure of the search engine's market share....
If I'm looking for an article on something and Google thinks Wikipedia is one of the solutions it displays it amongst OTHER results. I click on it because I tend to like their articles.
I say the stat is what the stat it says it is. Wikipedia visitors tend to use Google more than other search engines to get there. I personally Google something with "wiki" thrown in as a keyword at the end so I can get to the correct Wikipedia article.
I believe it's global, but if someone wanted to do the legwork to determine US only, all the log files are uploaded on an hourly basis here http://dammit.lt/wikistats/
Google represents at least 90% of search engine references coming in to that site.