In particular, one private party has an expensive and highly educated legal team and a lot of time. The other party wanted to eat a burger and didn't have a week to do a thorough legal review of the TOS to check if they were potentially selling any kidneys for a dollar.

US contract law has the concept of unconscionability already. You're not listening to me, my question is why should those things be allowed but nothing else? "Because a severe power imbalance allows for abuse, and governments should prohibit such abuse." does not address my question because it does not explain why the would-be permitted things are not subject to severe power imbalance or abuse of a type that governments should prohibit.