That's a bit like saying the only people who should wash their hands after using the john are those who work in food service. Just because the duty exists in a profession doesn't mean it doesn't also exist elsewhere.
The duty to disclose happens any time you want to maintain the trust of others in your objectivity.
Example: Say I'm having a beef with my girlfriend. My fictional co-worker Billy really wants to date her. If he gives me advice to break up with her while keeping his desire a secret, he commits a breach of trust. His interest in advising me soundly has now conflicted with his interest in hooking up. When I later discover this breach, the trust is eroded.
He doesn't have to disclose anything. But if he wants people to continue reading his blog while trusting his position comes from ideals instead of self-interest, it might be a good idea.
Hey, if you want to gain people's trust by disclosing your investments whenever you write a blog post, tweet, or comment on HN, go wild. I never said don't do it.
However, I reject the suggestion that failing to do so is like serving unclean food. If writing is like food, it is clean or it isn't and washing your hands is like spell-checking or fact-checking, not like disclosing your personal interests.
Otherwise, I accept the analogy that writing a blog post is like using the john. Or at least, looking at my writing, I see the comparison is apt.
The duty to disclose happens any time you want to maintain the trust of others in your objectivity.
Example: Say I'm having a beef with my girlfriend. My fictional co-worker Billy really wants to date her. If he gives me advice to break up with her while keeping his desire a secret, he commits a breach of trust. His interest in advising me soundly has now conflicted with his interest in hooking up. When I later discover this breach, the trust is eroded.
He doesn't have to disclose anything. But if he wants people to continue reading his blog while trusting his position comes from ideals instead of self-interest, it might be a good idea.