It isn't the forward speed of a fall that is really important but the downward force. Falling from a jump or rail will likely result in more than someone hauling ass flat based. Plus there is the issue of smacking your head on the feature. If I were riding park I'd be wanting more protection not less.
That's totally valid and I am by no means doing insane shit in the park, but in my experience doing that and skating, a hard hat has been just fine and, in a side by side comparison of having to use my certified helmet has resulted in less bell ringing.
Again, what you're saying makes sense, but the hardhat has been what has worked for me better.