But these studies are on noticing structural changes, not changes in the subject's capability.
What makes meditation better at this than, say, sitting down and doing math homework or going out and doing exercise? There's a lot of research to strongly correlate regular physical activity with intelligence (random googled summary with many citations here: http://www.unm.edu/~lkravitz/Article%20folder/brainandex.htm...).
Because medical professionals can believe in magic and bullshit just as easily as anyone else.