We do not allow this because it is not something SmarterTrack's event system is designed for. If you want to have emails directed to different departments you will have to use multiple POP accounts. This is not that different from sorting based on the TO and CC fields, and it will have a more immediate effect on ticket routing.
If the event system was configured in a way that could handle your request, it would increase work on the server for every ticket that came in. In addition, new tickets would end up in the wrong department for several minutes before they were moved to the correct department. This is because the event system does not look at any tickets until after they are created.