There are also plenty of stories of hardcore BDSM models who are willing and even enthusiastic about being beaten and whipped viciously, hung upside down, forced into oxygen deprivation by breathing into bags, and nearly drowned by being locked into submerged cages... but their one red line, their one hard "no," is being tickled.
The distinction, I think, it rooted in their involuntary reaction to being tickled. I think to them, even when being beaten to the point where they cry out with each blow, they're still in a way "taking it." The psychology is that they're proving they can handle it. But laughing, squirming and jerking, by contrast, feels to them like they're being owned.