Gack. People have thought art important for as long as they have been people. Remember the cave paintings?
For some things the rarity and replaceability determine part of the value. Can we get another Han vase? Not as easily as we could get one from you! In 1,000 years, one of your pots could be just as valuable as the one broken by Ai.
Interesting that the idea of ownership makes one ok and the other not ok. Destroying an irreplaceable object, especially one of some cultural significance, is still wrong in my opinion.
After all, it's not like anyone was unaware that the Chinese government has been destroying the country's cultural history wholesale since the 1940s.
But then, I get upset when musicians break instruments on stage at the end of a set. It shows a lack of respect for the makers of the object that I dislike.