I'm curious. Under what circumstances would it be _not_ possible to disable RC4?
Is this in case there is a Windows 98 machine running somewhere in the network?
I will make a slight subtle distinction though. Cracking a hash doesn't mean determining what the input must have been. It means finding an input that resolves to the same hash - not necessarily the original input.
Still, even if they are shorter, if you do find a hash pre-image you don't actually know for absolute certain that what you found was the original input. Collisions are still possible.
In practical terms this doesn't really matter mostly. It's just good to be very clear about what you actually know when dealing with cryptography.
They're just limiting the input to a "reasonable" range to find one faster.
Side note: A quick look through the iOS keyboard shows another 25+ characters that can be reasonably expected to match between operating systems. I need to try using some of them in my passwords. Going from 70^(length) to 100^(Length) is nothing to scoff at.