Bingo, that's the point. It's a statement on the value of art. At the end of the day, a painting is just a piece of linen with shit smeared all over it. You might as well just tape a banana to the wall and be done with it. But it also shows something else - art isn't just paint smeared canvas. It has meaning behind it and it evokes emotion.
So this guy eating it and pissing off 90% of people does the best thing that he could have done. Also, the remaining 10% know that it is not about the banana on the wall which was replacable. It was about the emotions - the outrage, the trolling. It is the same artwork - since art isn't just paint / just a banana.