I guess that's the point: For some people it might take years to finally reach that critical 25$ mark, but they might still want to put the earned money to some good use.
Also, some people might want to just use their money from tapas for charity in general, since they personally don't need it.
So yeah, sounds like an lovely idea I would get behind.
But yes, there would be questions like "To which charity would it go to?" should be thought about. One might favour different organisations with very different topics. So it's not too easy to resolve. Offering to transfer to any personally chosen charity on earth might make things overly complicated.