Depends exactly what they bought. Those liabilities might be owned by the remaining shell which is now in administation or a vapour-like "parent company" rather than having been transferred to fitbit.
Maybe not moral, but quite probably perfectly legal.
I was not involved in the structuring of this deal, but have been involved in deals like this one. The terms are very arduously worked through to make sure the asset acquirer isn't accidentally picking up a liability they don't want.
Regarding the morality question: consider the alternatives. The moral choice is in the hands of the seller. A buyer is an option among many.
Maybe not moral, but quite probably perfectly legal.