This is art, because it is about the idea. He conveys the idea very effectively through three simple images. He is saying, I appreciate the old, but we need to move forward, and make change. He is a revolutionary and fights against corruption and for everyone's freedom, not just the Chinese.
Yes, Ai Weiwei dropping an urn is Art, as Art comes in many ways, shapes, and sizes. Art is in the eye of the beholder, so if someone observes an act or piece or hears a song or poem and finds meaning in those things, then all of those things are pieces of art.
Yes, dropping an urn is not pretty colors on canvas or four-part harmony and it doesn't really pass the test of, "well I can do THAT." However, unless you know the context of the subject, the action, and the artist, one cannot truely begin to judge what is and is not art.