The only tax benefit you're giving to Americans is that you're levelling the playing field between the Americans and the Canadians, and the Americans would not have to pay capital gains, as Canadians don't now on ecological gifts. You have to remember this property would be coming back into Canada, at no cost other than this capital gains tax, because the tax credit on the value of the property will actually be looked after in the U.S. system.
It will be the U.S. government that will actually be subsidizing--if you call tax credits subsidizing--to have this land brought back into Canada under Canadian ownership and preserved.