This is the key. I can't help but imagine some hyperbole of the advertisement for participants reading some absurd thing like "Are you highly confident in your spiritual strength and have free time to earn $10 / hr?"
Are they participating to earn $10/hr or to help science and understanding of meditation?
On the other hand, why can't someone who is confident in their spiritual strength just be poor and actually need the $10/hr?
I'm not sure why these types of studies are taken seriously. A clever researcher could probably get any result they want.