Regional cuisine is fun and interesting. People who put a lot of effort into food probably have better ideas than the average joe. I think more people would get more enjoyment if they go to a restaurant and say "make it how you make it" rather than saying "I know how I like my hotdog and I want you do do exactly that."
Ketchup is a very overpowering condiment. It really does overpower everything else. If you're eating a dog and a bun then maybe it doesn't matter, but if you're getting a dog with various vegetables and seasoning on it, then it's kinda pointless if you then spray ketchup on it.
If an Australian showed up and decided that since they like the taste of vegemite they are going to slather it all over everything you might think to yourself, "he's just eating vegemite on various substrates". You might also wonder why he even tries different foods from different places if they're going to all end up tasting the same.
I someone knows how they like their hotdog, and want you to do exactly that, then it doesn't matter if you are 3 star Michelin restaurant chef. You don't know better. It doesn't matter if you devoted your life to the perfect hotdog and got PhD in hotdogology. You don't know better. If a person knows what they want and want it that way it is perfectly acceptable. It isn't acceptable to pretend you know better.
The australian in your example may just like the accents other foods give to his vegemite. They will all taste like vegemite, and if they like that, that is perfectly fine. You can wonder about their taste all you like, but that is just your problem.
Encouraging people to go beyond their comfort zone is fine. Thinking you know better than them is just arrogant.