The reason is quite simple.
On the bell 206A, at very light weight and aft CG, with the doors off, the aircraft has negative static stability - ie when you start from 60 knots, you have to push the stick forward to go faster, but when you stabilize at 70 knots the stick will be slightly farther aft than at 60.
Negative static stability was not permitted by the regulations, but the choice of words is awful - stick reversal is not a term that is used anywhere else, and leads one to think that if you are in that situation that pushing forward on the stick will bring the nose up, which of course is patently wrong.
The Canadian Air Force did some tests on the Bell 206BII, which is heavier and when fitted with radios and an interior has a more foreward CG. They did tests to 100 knots and found the handling acceptable.
Because of commonality, and a desire to keep costs down, the doors off supplement from the Bell 206A has been kept.
But I often wondered if some lawyer wasn't going to take Bell to the cleaners over the 'stick reversal' term.