Before the more modern definition took hold propaganda was not a term that held any real negative connotations. It was really just like marketing or evangelising, a behavior or product which uses various methods to change opinions. The big problem is when you have political or corporate powers using these tools to change opinion about something in a way that degrades democracy or causes harm.

For example, oil companies stand to benefit from people being uncertain about the science, so when they engage in propaganda they are trying to inject doubt where there is none and to do so in a way that will benefit them in the short term. This will cause massive harm in the future, potentially leading to a significant number of wars and a staggerig death toll, but that is not part of their consideration.

Another example is alt med. When someone claims that their pill can make your brain work better and will also boost your sexual performance all while protecting you from the dangers they just told you about they obviously stand to benefit from you believing them. They create the need and then offer the solution. Alex Jones is a good example of this. He tells you that the global elite are planning nuclear war, then in the next ad break tells you about iodine for radiation exposure.

So why is propaganda frowned on? It is more like propaganda is the label we give to marketing or evangelising that we consider worthy of frowning. Someone in another countries may sort different things into evangelising, marketing, and propaganda categories but they will do so based on their understanding of the world and you will likely agree with most of their sorting. Almost everyone thinks that Nazis are bad and put their marketing in the propaganda bucket.