Stuff like this happens all the time unfortunately - sometimes the media makes a big deal out of it, which appears to be the case this time, sometimes they don't. I am almost embarrassed to link to the rather inane reporting of Gawker and The Daily Mail, both of which sensationalize the story (but I did, so you can read more about it).
And then of course people on Gawker all have the knee jerk reaction of how poorly behaved American tourists are... which is very tiring.
We don't have a lot of 600 year old statues in America, and most people are not well informed about this stuff. Everyone is to blame (individuals, media, institutions, etc.). There is a 2,000 year old marble lion on the steps of the Loggia dei Lanzi. If you spend a couple of hours watching it on any nice day you will see tourists of every shape, size, color and nationality lean on, kiss, pet, and try to climb that lion - and that includes Italians. And it could be for a variety of reasons - ignorance, cultural differences, sheer naivety, etc.
The fact is that people need more information about these things - they have to be informed somehow at some point how to behave around art, artifacts, etc. I don't know why when you get off the plane in Italy and get your passport stamped the government doesn't stick a piece of paper in your passport explaining this in a simple way.