Turns out no matter how famous you are, once someone at a wedding knows you can sing, you will get pushed up on stage to do a number or two. Hence why everyone wants to try and be mates with Beyoncé before they get married, so they can dodge the performance fee.
Bono was at the wedding of Emily Greenberg, daughter of the Buffett family who Bono does loads of philanthropic work with. We're going to go out on a limb and say he did it for free, considering.