The Princes didn't organise this trek, Enduro Africa is the organisation that runs it each year. They learned about the trek when Sentabale was contacted to see if they wanted to be a recipient of the money that the enduro raises. Enduro Africa has made a commitment to Sentebale for the next 3 years which means money raised in the future enduros will go to Sentebale not just this year's. The publicity it's receiving due to William and Harry's participation means more people hearing about the trek and more people doing it and more money raised.
This was an relatively easy fundraising event for the Princes, Sentebale will get 100,000 pounds from this event, which the princes just joined. In contrast an enormous amount of work went into the Diana concert, including input from the Princes and it raised a one off approx 150,000 pounds for Sentebale. ( from the Sentebale site) For another 3 years without the princes having to be doing the fundraising approx 300,000 pounds will be raised for Sentebale. More money less work, so the trek was worth doing.
Concerts ( personally I think the Diana one was tacky) that are repeated aren't that effective, even the Princes Trust which used to have a concert a year in Hyde Park to raise money has stopped doing them. The novelty has worn out and they aren't cost effective.
The area where the trek is doing through is not seething with danger, that's why the trek is there! The danger is in people falling off their motorbikes and breaking bones. Not everywhere in Africa is a hotbed of murderous thugs out to get you, nor is everywhere politically unstable and about to break into chaos. It's grueling otherwise it's not an adventure!According to reports, the protection officers are riding in the enduro along with the princes ( as are some of their friends, I'm sure I recognised Guy Pelly in one of the photos) Also the other bikers are the 'weekend warrior' who are out for a taste of adventure. Reading one blog a rider has spent 2 years raising his target of 20,000 pounds for charity and for his adventure. They take a week's holiday and go to Africa, Harry's been given leave and William is between placements as he's been spending the last few months doing various placements in the armed forces, including a stint at Whitehall.