The event is set up so that you can buy either the hat itself, or a piece of accompanying artwork that showcases the hat. Or you can buy both! That's why the full name of the event is "Art in Hats - Hats in Art."
If you do buy some hats, and are having trouble getting them fitted with your hair, here's a tutorial that may be of help: http://acrossthegridwithlindal.blogspot.com/2011/06/hats-off.html
Here are a few pictures I took, but they're only the tip of the iceberg. DO visit this extraordinary event!