Benefits were not withheld from the union, they get to negotiate separately. They earned the right to negotiate. Most of the company is under at-will employment.
Non-union Apple employees didn't get to choose those "additional" benefits and didn't have a choice of taking the cash value or bargaining for something else.
Apple has a choice here - they can offer the benefits freely or withhold them to try to extract something else from a negotiation. If offered freely the union could still reject them, and try to extract something else instead at the next negotiation. If the union did that it would at least raise the question of whether the money spent on those benefits might have more utility providing something else for the workforce.
Non-union Apple employees didn't get to choose those "additional" benefits and didn't have a choice of taking the cash value or bargaining for something else.