Creation rights

Recently, I entered a discussion on licensing, specifically relating to usage of software applications. With the increasing popularity of Web 2.0 technologies, specifically web services, feeds, and mashups, it raised some curiosity in me.

Users can now create mashups of different sites. The content isn't explicitly hosted or created on the mashup site, but the mashup site aggregates the data from a number of other sites. What is the legal implication of this?

The original content (whether it be plain text, markup, source code) is not created on the mashup site. The mashup site creator does not own the content, but is merely collecting it from other sites similar to how a browser collects data from another site. If the content is permissive, then all is well. The authorship site (owner of the content) can give the ability for other users to grab content via feeds (RSS or Atom), web services, or otherwise if explicitly defined in the license for that content.

What if the content is not defined as permissive though? Take for example a random site that scrapes another web site for content and presents it as its own? An explicitly shareable content feed isn't used, but the content is parsed off another site in ways the original author did not intend to distribute his or her work. Is this legal?

What if the mashup site presents advertisements while aggregating content that they don't own or necessarily have permission for? What exactly qualifies as fair use in this context?