I think it's very important when you're using terms like “tax expenditure” to understand that while you deem it a cost to the treasury, it's difficult to argue that it is a cost if you view it as something that doesn't really cost the treasury directly as a tax expenditure. If there had been no contribution whatsoever in the first place, there would also have been none of the leveraging that leads to other types of activities in the philanthropic sector.
When you say “a tax expenditure of $34 million”, it somehow attributes a cost to the federal government. What if they never sold the shares at all?