The paper points out that many of the Ponzi contracts on Etherium have exploitable bugs. Some can be induced to compute too much and thus run out of "gas", aborting. Some allow an attacker to change the party who gets the fees. Some can be stalled out with a suitably constructed transaction.
Making Etherium contracts a full byte code execution engine was a big mistake. That form is too bug-prone and led to the DAO debacle. It should have been something simpler, such as a decision table.[1] That simple declarative form can handle most useful business logic, but can't loop. It's always decidable and is easy to hand-check.