Why is this illegal?

At first it was an amusing, slighly tittilating story.   At the Uffizi art museum in Florence, Italy, a young man stood looking at the Botticelli painting, "Birth of Venus."   Then calmly and without fanfare he stripped off his clothes and assumed the same pose as the naked goddess in the painting.

 
Other museums goers took pictures with their smart phones, which quickly circulated on social media.   Finally the guards stepped in, and he was charged with "indecent exposure."

But, as I thought about it, this bit of whimsy, or performance art if you prefer, became a serious question about social mores -- and laws.

Why is it "indecent" for a live person to stand naked in front of a painting of a naked person that the world has revered, and flocked to see, for over 500 years? 

Ralph