In psychology/behavioral economics this is known as loss aversion. Most people would rather avoid losing $5 than not earning $5. But given the rather broad view

In psychology/behavioral economics this is known as loss aversion. Most people would rather avoid losing $5 than not earning $5. But given the rather broad view of reinforcement learning this paper adopts, I would guess that the authors would not view this difference in sign as important. (And indeed, in the more ML/AI view of RL, viewing reward as a negative loss or cost is quite standard and for the most part the sign is seen as an arbitrary convention).