Fellowship is explained here, but is usually acquired by election following a peer review process, and the main criterion for election is scientific excellence. The Royal Society website explains that “Each year we award Royal Society Fellowships to 44 of the best scientists in recognition of their scientific achievements”.
“Prince William of Wales is elected as a royal fellow of the society in recognition of his developing leadership role at national level, his active interest in science and technology, and his ability to stimulate interest in science and technology in young people” said a Royal Society source. “There was no thought of changing the engagement but it’s just no-one thought the game against Slovenia would be so crucial.” said a royal family source.
Fellows past and present include Robert Hooke, Isaac Newton, Michael Faraday, Christopher Wren, Joseph Banks, Charles Darwin, Stephen Hawking, Richard Dawkins, and Martin Rees; and (among other members of the royal family) Prince William’s father, Prince Charles, Prince of Wales, FRS. His 6 O-levels, 2 A-levels (B in History, C in French), 2:2 Cambridge degree in History, and lobbying on behalf of homoeopathy, herbalism and and other “complementary” treatments (see Simon Singh & Edzard Ernst’s Trick of Treatment?: Alternative Medicine on Trial), make him an obvious candidate for fellowship.
William apparently delayed his arrival by 37 minutes so he could watch some of the England World Cup match, he had to get there before his gran but when you have a police escort it’s usually possible to leave things until the last possible moment. Getting his priorities right during the ceremony, according to some newspapers William received regular whispered football updates from his aides. It might have been more discreet to get Dec to text him.
But after all, there is a “Royal” in Royal Society. The initially unnamed society was founded in 1660: Sir Robert Moray approached King Charles II – his approval was forthcoming (but not, I believe, the hoped-for cash), and so in its second charter the society was named The Royal Society of London for the Improvement of Natural Knowledge. This sort of thing was common in those days when sycophancy was the norm.