But isn't the joke either (or both)

A.) The fire-walker in question is too stupid to know that halfway back is the same as halfway there, so we laugh at their foolishness
B.) The fire-walker is going to suffer the same amount of foot pain as they would have if they'd walked forward, so again we laugh at their pain/foolishness.

No one real is hurt, but you are still laughing at the idea that someone will not be having the happiest outcome possible (i.e., jump off the pathway to the side, end of fire-walking experience--not really so funny.)

Well even if they jumped off the side they'd still have burnt feet.

For me the humour is in the history of turning back when things get a bit rough. In this instance, the usual solution which is to turn back is just as bad as going forward. Also, as you say, the people are not real. It's a hypothetical situation. It was also an interjection into a serious question, the answer to which happened to be that it's never happened (although I doubt the legitimacy of that answer).

You can't get through the forest, so you go back the way you came. Hansel and Gretel wanted to follow their bread crumb path back (and we all know how that turned out).

LoL that's funny.

All humour has a 'victim', the subject or butt of the joke, but not all humour is cruel.

I've heard that scientist define part of humor as the whipsawing of the mind -- when you expect one thing and get another. The more unexpected, the greater the humor. Thus the "brown and sticky" joke. That does mean that the "cruelty" is towards the listener, in as much as they got jerked around. But they're laughing; they must like it. It's the mental equivalent to the rollercoaster.

Therefore, discussing humor in the context of cruelty must include the quality of the response to the joke. It's only cruel if someone found it offensive or unethical or hurtful.

:. Know your audience.

I think that sums up what I was trying to say.

I suppose my brother and his wife could have been offended at the jokes my husband cracked about firewalking, but they didn't. They laughed because he wasn't trying to be hurtful towards them or firewalkers and he wasn't making any judgement about how smart or stupid you'd have to be to do it.

It was funny, and if you'd been there, you would have laughed.