That's not the case in many (most?) of the cases I mentioned. In plasma for instance, the operator and seller could collude against the buyer but any "illegal"

That's not the case in many (most?) of the cases I mentioned. In plasma for instance, the operator and seller could collude against the buyer but any "illegal" operation they do on-chain is enough for anyone to trigger a mass-exit and cause financial and reputational loss to the operator. A common theme in all the on-chain payment technologies I mentioned is that they do not put any one party in "charge" but the volunteers who run staking channels of any sort (plasma, lightning, etc) can and will suffer financial damages (loss of deposit, etc) if proven to act maliciously.