People often ask me, can I take a Google Maps mashup, and make a KML file out of it (or in some other data format)? This question never comes from a Maps developer, only from people who see a cool mashup and want to overlay the data on their own map.
The short answer is: no, you can't.
The long answer is: no, you can't, and that would be wrong.
The longest answer is: no, you can't, unless the developer chooses to make a KML file available to you. It's possible, in fact, that they are using the KML file loaded onto their map using GeoXml or some other method. Otherwise, no, and it would be wrong.
First, from a moral point of view, the construction of that site belongs to the developer of that site. Usually, the data isn't presented to them raw, a certain amount of development has to happen to transform the data into a format usable for the app. In fact, they may have agreements with the data provider that they don't, or some license in the download from a public site restricts redistribution.
Of course, it's possible they just didn't think of it. Ping them, maybe they would do it for you just to be nice. Depends on the data of course.
I'm not saying that people can't look at your code and figure out your data sources, if you're the developer. If you want to protect that data source, you should design accordingly.