In my opinion it is kind of presumptuous. We generally don't think it is acceptable to poke our nose in other people's business elsewhere, but somehow it is kind of okay at the golf course.I really dont know why people take so much offense to advice givers. I know some can be a pain in the but. However its only rare from my experience and other than that i think that most people do simply just want to honestly help especially if they notice something. A freindly rsponse like "thank you but I am working on what my instructor said" or "thank you, I really appreciate it but I'd rather do my own thing". And usually you'll just have a very friendly enviornment after that.
I do agree that short game technique isn't overly complex and lesser skilled players can have a solid understanding of it. But I think that knowledge is only part of the skill set for instructing people.