The Vne for an external load is usually set by the response of the aircraft after pickeling the load, similar to the response to a SAS hardover. For most helos, there is a strong pitch up when the load is released. This is because the reduction in gross weight looks like a collective pitch up, since the collective is far too high for the helo immediately after the release. This sudden reaction is a tested condition. That being said, I doubt that the 350 does anything wild at 80 knots upon load release, so it could certainly be for the reason that the manufacturer didn't test further.
The max speed is seldom set by any structural parameter while dragging the load, since most external load Vne speeds are quite low relative to the normal Vne, so that the basic stress on the rotating components is low.
One must remember that the actual max speed for a given load is often set by the load's behavior, and all loads are different. We test with a block of concrete, the most benign of all loads, so the actual speed with something else (like a pallet of feather pillows) will almost always be less. Load swing, skating or bouncing are al signs that a slower speed should be used.