To demonstrate this ability, what conditions are the manufacturers using? Max cruise power?, Max AEO hover power? Power at Vy with MGTW?
Not sure what manufacturers use, but the FAA demonstration requires the minimum torque necessary to sustain flight at the maximum gross weight and the most efficient flight conditions. The torque is increased at 25 seconds remaining and at 10 seconds remaining to simulate the loads of a landing. A small amount of damage is allowed, as long as autorotation is not affected.
You do bring up a good point in that when flying beyond 30min of a safe landing spot, you're committing yourself to a ditching. However, there are other failure conditions in FW and RW flight where flight cannot be continued to a safe landing area. A risk that has to be accepted or the flight can't go.
Cheers,
Matthew.