Scientific experiments, as those conducted in these studies, test for a "null hypothesis." In other words, they come up with a hypothesis (in this case, saying that video games do not diminish prosocial behaviour) and then seek to prove the opposite.
In this way they are considering the possibility that video games actually do diminish that behaviour, and test for it. They were unable to prove that video games diminish prosocial behaviour - they failed to prove their null hypothesis, leaving their primary hypothesis correct.