Odysseus' men have been hungry for a month. They argue that they are going to die from starvation if they do not eat the cattle of Lord Helios. That death by starvation would be a painful and slow death. Thus, if they ate the cattle, not only would their tummies be full and satisfied, but their death by Lord Helios would probably come about more quickly. Their rationalization had everything to do with dying in the most comfortable way possible with the least opportunity for suffering.
This occurs while Odysseus is praying away from the men. Therefore, he has no way to stop them although he had warned them not to eat the cattle prior to going off to pray.