It may be due to this "mast bending" I remember for instance that the AS355 had speed limitations in relation with long line length, I think the slowest was around 30Kts with a 200 feet line.But it seems weird with articulated rotors...
In this case, don't you think the torque instead of speed is rather to be considered ?
On older helicopters I have flown (lama, alouette 3...) the limiting speed was based on load behaving.
I have witnessed AS 350 B3 currently flying their loads or empty lines/long lines at more than 80 Kts (.... myself included I should say) especially without load.
Beside the long line behaving, another thing we had to take care, was to avoid the cargo-swing frame touching the hull. Normally shouldn't happen on smooth normal ops.