Isn’t propaganda just a form of persuasion? What about propaganda separates it from advertising or interpersonal communication?
Edit: Not all propaganda involves lying. For some reason, we seem to be more comfortable with a person lying to us than we are with viewing a propaganda poster that uses verifiable facts.
Edit 2: Another interesting note is that in some countries, propaganda is not viewed negatively like it is in English speaking countries.
It isn’t frowned upon when we’re selling a product. But we call it “advertising” then.
On second thought, everybody hates advertising too. Except for the people who like it.
But to answer your question, it’s frowned upon because it’s disrespectful.