Their job is explicitly stated in state laws. If they vote for fraudulent votes, those votes have to be decertified. Not all states have identical laws, but many are identical.
The overall federal system sanitizes fraud. The handoff from the several state governments to the federal government is the moment in time when questions about validuty, legitimacy, and potential fraud need to be asked and answered. If that transition occurs and passes without substantial incident, and the federal VP accepts the votes, the process is basically over (pending inauguration day). No backsies!