Going back to the charity/conference topic for a second, the concert is an event for conference attendees BUT it's also being attended by big name donors to the Children's Hospital. The conference attendees get a free gig as part of their badge and have the option to donate, learn about the cause, etc., but there's a whole separate viewing section, entrance, and after party for high-wealth/non-conference attendees that are paying premiums to see U2 and support the cause.
These two mutually exclusive groups of attendees are essentially sharing the gig with Salesforce CEO Marc Benioff being the common denominator. Benioff is a super generous guy and supports lots of local San Francisco charities/schools/hospitals and this has become one of his premiere fundraising events.
So U2 playing Dreamfest makes sense because of their Salesforce connection, the Benioff connection (Bono and Oseary are quite close with him), the (RED) connection, and because of the UCSFB Hospital charity event.