Hey there, yet another American here. Just have to say that not all of us live on canned and processed foods! Another "ugly American" myth!
As you can tell from the other two "Yankees"* on here, grits are kind of a divisive food in the US. They're essentially watery polenta (but white), or watery "pap" (not sure if they have that in Australia--it's a South African thing). It's almost exclusively a Southern thing (Texas and all points East--staying south of Washington, DC). Southerners seem to love them, everyone else either thinks that they are bland or strangely textured, or just hasn't even heard of them.
I think "pork grits" is a confusion with "pork rinds", and I just don't know what they are except that they're generally considered very unhealthy and I think are also associated with Southern cooking.
Canned pumpkin, though, I must defend! I think we may talking about different "pumpkin" here. I've never lived in Australia but I have lived in English speaking countries abroad and it seems that what's called a pumpkin there is a different variety of squash than what we call a pumpkin (usually a butternut squash outside of the US). Even if we're talking about the same pumpkin, the canned stuff is used to make pies and pudding, not as a side dish for a regular meal. It has a flavor sort of similar to spiced sweet potatoes, put into a pie crust and baked.
It's not terribly uncommon to preserve pie filling (for any pie) by traditional home canning methods--it's like canning homemade pasta sauce, relish, jam or salsa--everyone has a different method and it's fun to share. Preserved pie filling can be found in most farmers' markets I've been to. So this canned pumpkin thing is like that but on a larger, commercial scale.
LIBBY'S Pumpkin History - NESTLE VeryBestBaking.com
Doesn't seem all that unusual to me, but I was born and raised here. Vegemite and Marmite I have tried, but cannot understand. Same with blood sausage/black pudding and sour milk. Oh, and prawn flavored potato chips/crisps and anything involving the use of sorghum (although that may just be personal taste on the sorghum--we use that for cattle feed here).
Everywhere has it's own "thing" (or several) that's hard to understand, I guess.
*BTW, even though Yankees was the term used for the rebelling colonists during the American Revolution and you'd
think it could apply to all Americans, it was also the term used pejoratively against Northerners by Southerners during the US Civil War, and that's carried over into the modern North/South divide and occasional animosity.
Every Southerner I know gets pretty prickly when told that they are a "Yankee" in the eyes of world so of course when they call me a "damn Yankee", I remind them that they are technically Yankees too. It's a great game ;).