That's the whole point; plausible deniability.
Evidence includes the fact that it only ever seems to be women (the stocks are manned by dudes), the stocks themselves look more like something a bondage enthusiast whipped up in his basement as opposed to an actual medieval prop (toe ties? really? Yes, I'm sure actual historical stocks had toe ties.
), there's usually a whole host of historically accurate tickle tools like electric toothbrushes on hand (again, something a tickle fetishist would have), and the big one, the fact that there always seems to be an accomplice in the crowd filming these things for fetish websites.