Slightly different fail in the Newton's case. The tablet/PDA/smartphone/MID/palmtop market was niche at best and effectively non-existent at worst when the Newton came out; it simply was in too small of a market to go anywhere. The PlayBook, OTOH, showed up in a crowded, relatively well-established market, and so far has almost totally failed to get traction with either developers or customers.
The Newton was supposedly profitable for Apple. It was killed not because it lost money, but because it was overall not a good use of resources for Apple despite making a modest amount of money.