They can scan the timeline like sensors scan in space, so they should know of any meddling by other time travelers, like Nero
But to them it's already happened and is history. They can only police time travelling from their own time period. If their sensors indicate a change in the time line then clearly it's been done by someone from their time period.
They were eager though to go back in time and fix Voyager getting blown up by a temporal weapon, they certainly intervened there.
If Voyager was destroyed, which it was, then wouldn't that have been their history already at that point? THEIR present was the present where Voyager was destroyed in the past.