There is a reason if you, as an adult without a child, are happy to be lumped into a category with morons as someone not to be trusted to ride in a boat for no good reason.
What if the stupid people had all been black, of oriental, or gay etc. They, quite rightly, would not say 'well, lets ban all blacks/orientals/gays from the ride', would they? But adult without a child is​ an acceptable group to declare unfit? Why?
And I said it was a dumb example, but it serves a purpose demonstrating a point: this rule does not get rid of the stupid people by the restrictions on it, but does stop a hugely disproportionate amount of perfectly capable people from riding.