It's to do I think with the idea of tickling as "torture" and the attraction to an idea of true helplessness.
I think there are some differences however when we're talking stories or videos, since stories feature fictional characters, videos are of real people, even if they are acting. In videos I like tickling to look enjoyable and sexy from both sides. I don't like seeing tons of screaming and upset, scared or angry reactions and facial expressions from the lee, even if these reactions are all scripted and the scene was all agreed upon and no real distress occurred. I can put aside my empathy a bit more for fictional characters in print stories, I guess, than when seeing the real face of an actual person on camera who appears to be in pain, extreme anxiety or upset.
However although I can enjoy more sadistic tickling stories I have also really loved ones where the lee loves being tickled, usually secretly wishes for it, and wants it from the start. My favorite stories too are always the ones where even if the beginning fictional scenario is non consentual, by the end the lee loves it, wants it, and gets off from it.
In regards to stories too, I think you'll notice that non con might be pretty popular on the fiction stories board, but compare that to the ones on "True Tickling Stories" sometime. If it's so popular in reality, you'd think this board would be full of "the time I took things too far with my girlfriend/boyfriend" borderline non con stories... But instead, it's nearly all describing innocent tickles that are often brief/playful, sessions between consenting partners, or sessions with professional dommes or kink models. Almost never do you see a true story there about pushing or disrespecting someone's limits, and if you did it would surely get backlash from the rest of the community. Would that non-consentual feet photos or videos got the same backlash here! 🙄
An interesting question too is why would so many people be looking for a consentual tickling partner on the tmf who also enjoys it if everyone mostly wants non con? I think nearly all the non con stuff while popular in fiction, is purely fantasy for most and doesn't reflect their real desires for a real scene or session.