The journalists are using term 'so-called state' to prevent controversies about the word 'state' which is used to describe a geopolitical unit. ISIS as a government is not acknowledged by any western state (they are not even called 'partisans' or 'rebels', instead they are referred as 'terrorists').
Using the term 'Islamic State' without 'so-called' could be interpreted, as if a specific journalist would consider they as a government that at least could be a candidate for acknowledgement. Saying 'so-called' makes it sure that you distance yourself from considering them a legitimate government, or sympathize with their claims.
In international politics, you need to be very careful about the names.