Mashups are not all one thing. Sometimes it's a lone hacker modifying their own data, so licenses are not an issue. Sometimes it's data that the authors intende

Mashups are not all one thing. Sometimes it's a lone hacker modifying their own data, so licenses are not an issue. Sometimes it's data that the authors intended to be shared. And sometimes people are just retards and they steal or misuse other peoples' stuff. Flickr is all about sharing and it tries pretty hard to give the users tools to control access to data and to express their licensing intent. For example, as much as is possible, there is simply no trace of private data in searches that don't have proper authorization. For public data, it is not a given that you can republish or modify the work. So the atom feeds have links to the licenses for each photo or video, and the API has similar features when obtaining photo info. Users are able to grant a mashup app access to their private data in a formalized way (and to revoke it later). Finally, Flickr is also able to revoke the rights of any particular app to download data or simply throttle them to a reasonable amount per day. All sites that offer RSS or an API should do these sorts of things. (The oAuth standard is a formalization of some of the techniques that sites like Flickr use.)