This sounds absurd. This is the best analogy i've come up with: A Sandwich shop sells half sandwiches at 5$ and full sandwiches at 6$ You, and a friend would both like half a sandwich. Instead of ordering two separate halves. You order only 1 full sandwich and cut it in half yourself. Now the sandwich shop is suing you because you paid $6 for one sandwich, and only ate half of it. That's instead of paying 10$ for two…
For the passenger, the contracts say that you are expected to use all legs of the tickets and if you do not then the rest of legs of the flights are voided. For the airlines, if they are unable to provide the flight they owe the passenger compensation.
There are no legal repercussions for the passenger not using all legs of the flights, but Skiplagged is clearly committing conspiracy to defraud and I would wager a fair sum of money that they will lose this case.
The issue with the Sandwich analogy is that there is no contract.