The STC can be issued by a region where the long-term effects of the changes are not within the expertise of the approver, an unfortunate aspect of the STC system. Manufacturers and original certifying regions are often frustrated by the degree and scope of STC work that sometimes violates the original design intent. The latest FAR approvals require that documentation as to the critical aspects of the aircraft (for future STC purposes) must become part of the certification data package that the manufacturer files with the other data. The S-92 has such a data package, I know of no other helo that yet complies.
This letter from Bell is literally a washing of hands of the home-grown changes that they are seeing. Those who want to read NASA reports or STC providers web sites might write those other folks if their tail cone falls off or the tail rotor system develops cracks or fails, Bell is now on record as telling you that the continued safety of the installation after such unauthorized modifications is not their responsibility.