Would you all rather she just not donate anything? Damned if she does and damned if she doesn't
She probably doesn't get to decide really, she can't dictate how much of the ticket cost gets donated. That ticket price goes to so many different places (the venue, ticketmaster or wherever, touring costs, the band and dancers and Katy, the crew, etc.). I'm sure she's donated some of her own money to UNICEF considering she is an ambassador for them.