Yes. If I don't trust the charity to decide how best to spend $50, why should I be willing to give them the time of a person who is paid that much? It's not like the charity itself can't use Exec to purchase the same services.
I agree, I think this unnecessarily limits the charity's options. Plus, I would assume that you don't actually get $50 worth of charity work: Exec takes some, travel time, etc.
Buy volunteer time?
i.e. hire people to work for the charity—probably at a significantly cheaper rate than $25/hour.