Judging from the age range (and from a previous life as a neuroscientist) they're probably undergrad and grad students who signed up because they saw an advert on a college notice board and thought it would be cool to have a brain scan. There's big issue in recruitment of subjects where many, many studies use WEIRD people as subjects - Western, Educated, and from Industrialized, Rich, and Democratic countries. It costs a hell of a lot more if you want to recruit a broad section of subjects. I'm not sure that participant re-use would be a big issue here because it's an anatomical study, unlike other studies where subjects might have learned skills from previous rounds of testing.

However, they do say in this paper that "Control subjects had no previous experience with meditation or similar practices." They're careful not to make claims in the paper that the mediation caused the changes in brain structure - "our findings suggest that long-term meditators have structural differences in both gray and white matter" - which point to exactly what you were suggesting as a next step: take a group of people who've never meditated before, split them into a meditation and non-mediation group and follow up after a year or two.