Afterwards, having had time to think a bit, it all made perfect sense: there's nothing wrong with anyone playing simple music beautifully, irrespective of their perceived 'ranking' in the world's hierarchy. For one thing, there would have been students present in the audience who were studying the piece and who would have benefited greatly by watching an acknowledged master interpret it.
But to many, 'simple' means 'demeaning'.
It's probably something that many of us forget. In many ways, we want to take on some complex, difficult music to mark out our territory – 'Look at me, I can play this...'. And it's quite probable that the real reason behind it all is that our own insecurity demands it. An opportunity to psychologically rise above the rank and file and take up residence in the exclusive and hallowed domain of technical brilliance.
John Williams' performance that night made me realise that the real trick is to be at peace with your own level of musical accomplishment at which point you are truly, spiritually free to play music without any consideration given to its technical demands.
And in so many ways, that state of mind is more difficult to reach than any advanced technique...