Besides, nothing is stopping them from giving more if they really feel that strongly about it.
That's not how game theory works. Toll roads do not work with voluntary tolls. A few might pay but the majority would not. Funding of public goods works best on a non-voluntary basis. That's why civilizations evolved things like "taxes" pretty early on once agriculture started taking hold. Everybody pays, everybody benefits. Yes, you effectively contribute a portion of your labor to the city/state, but if everything is balanced right, the rewards of cooperation exceed the cost of your labor contribution.