There was a message put out about a year ago -- not sure exact date, my memory fails me. Basically, the worry was that people are out buying seat cushions from anywhere and everywhere -- and NAVAIR hasn't looked into it.
As for why, (from what I understand -- am not a doc), the cushion would cause your body to move separately from the seat stroke -- so when the seat bottomed out -- you stood the chance of screwing up your back. Kinda the opposite deal of an ejection seat going up... In reality, I just think that the NAVAIR folks haven't really looked into the issue, so they banned them all.
In reality, I don't think they are necessary -- even with the kevlar bucket seats in the H and the S.