The risk is that legislation can have unintended consequences. Once there's a law specifically to control what can be published and what can't, on top of a multiplicity of existing laws (confidentiality, defamation, contempt of court, Official Secrets, being a party to misconduct in a public office and many, many more) then it may well be used by those who fear being exposed or embarrassed to choke off perfectly legitimate investigations. If it's dangerous legally to pursue an investigation, hence extremely expensive, proprietors will want them to be dropped, and journalists won't be allowed to even look for them, let alone report them.
Investigative journalism is expensive and time-consuming. Publishers are wary about spending money on stuff that could get them into trouble.
There is also the argument that celeb tittle tattle subsidises investigative journalism. Gossip sells newspapers and magazines which can then put money into serious journalism. Take the gossip away and the newspaper or magazine folds.
I used to edit an investigative magazine. We had every page read by a libel lawyer before going to press - something that cost a huge amount of money and is a luxury most publications could not afford. We'd still get shysters sending us threatening letters and trying it on (and politicians using PR spin - paid for by the taxpayer to try to shout us down). I was in the fortunate position of having the time and the legal backing to be sure that we'd conducted our investigations properly and ethically and that we could justify what we'd written. There's not many publications that can do that and indeed, that magazine eventually closed down partly because the finances didn't stack up (the publisher decided to invest their money in the main title and close the smaller ones).
I like to think we did some good work, certainly exposed quite a lot of wrongdoing, but the number of publications that are able to do that securely, knowing they are able to defend their work, is shrinking all the time.