I dont see how requiring credentials for a job in any way affects free speech. I'm not saying you cant write something down if you dont have a license, if you want to start a blog, magazine, newspaper, etc without a license on you go, write whatever the hell you want.
Its about setting standards and showing that you have a seal of approval from a governing body. Think of standards compliant HTML/CSS and the W3C, it sets the standard, but you're completely free to ignore it and write bad markup, but it'll be recognised by your peers that you dont meet the standard. THATS what i'm talking about.
THAT wasn't clear. Both of your examples, law and medical licenses are legally required.
Journalism schools already teach ethics and offer certificates. Reporters already are members of organizations with ethics statements:
http://www.spj.org/ethicscode.asp
All I'm saying is legally requiring some sort of enforceable code of conduct for journalists before they can work is a violation of free speech.
Its about setting standards and showing that you have a seal of approval from a governing body. Think of standards compliant HTML/CSS and the W3C, it sets the standard, but you're completely free to ignore it and write bad markup, but it'll be recognised by your peers that you dont meet the standard. THATS what i'm talking about.