Since journalists report public affairs their loyalty should be to that same public. We are after all bringing the public news about the public. If journalists don't answer to them (the public) who else would they answer to? I suppose they could answer to individual media companies but they would eventually have to answer to the public as well.
Public relations is different. They do not bring news to the public about the public. They bring news to the public about one specific entity. Their job is to report one business or person to the world. Since PR practitioners have defined and specific clients they should be loyal to those clients.
The difference between the two I believe is this; PR practitioners have the responsibility of representing their client well - to paint a good picture. Journalists, however, serve the public and sometimes the public is best served by pointing out the negative that needs to be corrected within that group.
I have my personal loyalties but my professional journalistic loyalties belong to the public. Being a 'wannabe' journalist, if I'm loyal to anything outside public interests I will have failed at fully representing my 'clients.'