In some cases, the decision to edit is purely financial: the owners believe that they can cut costs by reducing the running time (so as to fit onto cheaper media) or that distribution channels will be more willing to buy an edited release.
There are no "rules" or "policies" that give cut and dry reasons for why something is edited. There are plenty of examples of things being cut that no one can give you the slightest rational justification for. Conversely, there are plenty of films that are released that seem begging for a prosecution.

