Why do shield laws exist in individual states but not at the federal level?

A shield law provides statutory protection for the “reporters' privilege” — legal rules which protect journalists against the government requiring them to reveal confidential sources or other information. ... Some states have statutes, known as “shield laws,” and some provide the protection through case law. Some states have statutes, known as “shield laws,” and some provide the protection through case law. But, there is no federal law to protect journalists from revealing their confidential sources.

Currently, as of february 2016, an american traveling in france would be by the exchange rate. the weakness of the euro and the strength of the us dollar means that items in france cost less to an american than they otherwise would. of course, if this trend reverses and the us dollar weakens and the euro strengthens, such as was happening in early 2008, an american traveling in france would have an adverse experience because items would cost relatively more.