Simply because the manipulation checks aren't always properly formatted does not mean there is fraud involved.
Generally, I've found the surveys to be legitimate, even if I would structure some of the questions differently on some surveys. But that's a design issue, not a legitimacy one.
Indeed, the other day, I participated in a follow-up survey to one of the ones on here. The market research company contacted me. For participation, I earned a $50 Amazon gift card.