The problem is related to how the wheels handle in choppers that have them. It's easiest to describe it via example.
Using UH-80 Ghosthawk and standard flight model you can land at any speed as long as your helicopter lands straight - it's just going to roll smoothly on the wheels, slowly loosing speed.
That's not the case with Kajman and some other helicopters with non-retractable gear. They just get damaged once speed is greater than 60 km/h.
UH-80 works perfectly, while other choppers get damage and eventually explode for some reason, instead of just rolling on the wheels like UH-80 does.