An omission bias occurs when both an omission and an action can lead to a loss. In this case, the omission is always opted for, because the damages caused by it seem subjectively more harmless.
One way to tell if a media is using the omission bias is to concource a large number of different sources and compare information from those sources with that information the media is reporting.
That is the best way to determine because you will consult other sources or information and compare them to notice if there is any lacking information. By doing that you will be more aware of a media is trying to change the focus of the notice into one that fits more ideologically with them.
The Persians were much less democratic and usually had local rulers that ruled with an iron fist while working for the main emperor. Had he beaten Alexander, chances are that he wouldn't allow religious and cultural freedoms in Alexander's territory the same way that Alexander allowed in conquered Persian territories.