Jimmy Buffett is wearing a costume to promote a "life style" that is actually about selling a brand, and products. For example this margaritaville brand BBQ ranch salad dressing pictured below. He used to be cool (also see picture below), but somewhere along the way, the "beach" thing started making some money, so he went with it and started wearing shorts.
The guys in AC/DC all wear pants with the exception of Angus, who again, is wearing a costume. He's not wearing shorts because it's humid. It started out as a gimmick and is now part of the AC/DC brand.
As for Robert Trujillo, a 50 year old man wearing baggy shorts and white athletic socks... I got nothing. I think he looks silly. To me those are clothes you wear at the house on Saturday.
You are right about the "dress appropriately" part... but my question is when are shorts appropriate. For me, the answer is at the beach, participating in sporting events, and in the home. Not on a stage, during a performance, in a professional working environment, in front of a crowd (who probably paid to see you).
Please remember that this is all in fun and IMO, and IME. No offense intended! Wear the clothes you like, play the gear you like.
If you don't care, you are happy with where you are at, and you are all about comfort, then wear what you want... but for me personally, I don't want to see a performer who doesn't care. I get it that you can care about the music and still wear shorts,,, but in spite of how we would all like for it to be, it's not really all about the music.