Taxes and Tolls?

Road tolls are use taxes. They are designed to raise money from the people who most use a government resource. Although the road could be paid for with any type of tax (and part of its funding does come from a general tax) most people consider use taxes to be a fair way of apportioning responsibility for some government services.

Because whoever they give the rights to build the tunnel basically wins the right to make money out of it - therefore the government doesn't have to pay a cent.