Designed to come off your head. You don't put on a helmet and expect it to never come off do ya? Perhaps since its on type it's difficult to articulate. You put a helmet on, you take a helmet off. They are designed to slip on and off your head. They are not designed to lock down your head and never come off like its some space helmet that is pressurized. See what I'm saying?
Designed to come off=removable