i know what you mean, but because a lot of coins eventually corrode in water, it damages historical sites and monuments for there to be handfuls of copper, zinc, brass and even silver thrown in with the bacteria and acids of human hands to oxidize down at the bottom of a water feature. they appreciate the donations to help sustain the site, but the damage a dirty penny can do to ancient marble is much more than one cent can support the site.
this is also the reason why ponds and fountains at public gardens often have signs requesting people don't throw coins in. it introduces human germs, startles and stresses out the fish, and messes with the acidity and oxygenation levels of the water, which can make it unhealthy for plants and animals. cleaning coins out of inhabited ponds and other natural features requires human intervention, which wastes a park's money, stresses the fish out even more, and can lead to footprint erosion at the edges of the waterway, too.
throwing a penny in a public fountain is a wonderful and magical tradition, but it takes a certain type of fountain: modern built, self-contained, not a protected monument, not a natural water feature, and not home to fish or plants. please be thoughtful about when and where you make a wish, and why an organization might ask you not to.