This is one of the shortcomings of the effort to keep the system simple.  In SFB crippled ships turn modes get a little worse IIRC, but they are not prohibited from turning.  When porting things over to ACTA this got missed.  This applies to ships with turn > 6" as well as crippled ships (double turn mode and 6" max move when one or the other would have been fine).  Our group is considering trying to playtest a house rule to allow tracking remaining straight movement from a previous turn (using a 5mm die placed on the ship's base).  Has anyone else tried this?