Whoever owns the fountains owns the money.
If someone throws money into a fountain and then wishes then that in itself is a complete transaction with whichever supernatural entity is being bargained with.
I am assuming that any entity which is powerful enough to grant wishes is also powerful enough to hang onto the money if that should be its wish.
Alternatively it's a quaint custom that people engage in as a form of custom and the pleasure comes with a link to the tradition.
The periodic clearing out of the fountains is both necessary maintenance and a form of income that pays for said maintenance. No one is really harmed by this.
Precisely. ๐