The problem isn't that the event is being organised by the Queen's grandson, but that her grandson's company are charging a fee for organising the event.
Why on earth have those representing the Queen's interests not warned about the probability of a perceived conflict of interest and sought to avoid that criticism. Transparency of a competitive procurement would have helped. Alternatively her Grandson could have acted as a specialist advisor to a different provider free of charge.
It makes little difference to the perceived conflict of interest that the event will be run on a not-for-profit basis since there is no means of demonstrating that the costs are reasonable.
But there's another procurement angle; whoever delivers this once-in-a-lifetime event will gain unrivalled commercial gain from the ability to sell services to future clients on the strength of the demonstrable experience gained. Given that value to potential providers, those procuring the event could have asked, "Is it really necessary to pay someone to organise this at all, would good providers see it as having such long-term value they'd have done it for nothing or even paid for the privilege?"
It never ceases to amaze me how often perceived conflicts of interest in procurement make the headlines when they could so easily be predicted and prevented. It never ceases to amaze me that there's an assumption you always have to pay?
PS. 10 February 2016: Queen's grandson quits birthday charity due to conflict of interest!!!!