The reason politicians require a conflict of interest act is because they wield power that can be used in self serving ways contrary to their jobs as representatives of the public.

How is that not the same as political reporting? This is our source of info...

If the source is tainted and that source is used to influence the public and elections (i.e., democracy) then those who provide the source of information too wield power that can be used in self serving ways contrary to their jobs.

Are reporters / columnists being paid for their articles by outside sources? By whom? Are they friends with members of a political movement? Do they have personal grudges against certain groups or individuals? Is their work being influenced?

Everything a politician does is judged, rightly, through the lens of public good vs personal or political gain.

Though we completely ignore this aspect of reporting and just trust the journalists/columnists to hold themselves up to integrity.

Technically we have ethics in journalism though I have never heard of it actually being enforced nor have I ever heard of investigations of undue ties a columnist may have that would unduly influence their work.