I understand that as time goes on people take ideas from other areas. The fact that rice is very popular in South America is a great example, it grows well there and is cheap so it satisfies the need. Imagining Mexican food without rice is difficult. I also understand that if I'm in Seattle and I want to have a French restaurant I'm probably going to cook Salmon because it's readily available and the locals like it. Sometimes you can get food from one culture in another because people request it. An example of this is the amount of soy sauce you find in Thai restaurants. I've been in Thai restaurants and had someone say "what kind of Thai restaurant doesn't have soy sauce?". That's like asking what kind of Polish restaurant doesn't have Italian food.
I was at the annual Seattle Night Market in Chinatown/ID last night and took a photo of this food truck. Now granted, the trucks name was Fusion on the Run but I think we're stretching the term a bit when you have Banh Mi, Tacos and Macaroni Salad in the same truck. Reminds me of the old joke - A Vietnamese man, A Mexican and a white guy walk into a bar... Never mind.
My issue is not that you can't have different kinds of food from the same truck/restaurant but that the odds that the cooks will know and understand each different culture and be able to do a decent job is small. The only way I could see this working is that if you actually had three different accomplished chefs that decide to do what they do best and combine their efforts. I don't think this is how these things come together though. It's usually an entrepreneur trying to figure out what his customers want and providing for that.
The Blimpies Sub shop near the Everett Mall is run by people from India who make burritos. I was talking to the owners and they relayed to me that Mexicans come into the shop and tell them that they don't actually eat burritos in Mexico and yet when you go into a Mexican restaurant there they are - burritos. I rest my case.
Fusion keeps life interesting and we can be thankful for it otherwise Thai and Indian food would be bland, Italians wouldn't have red sauce, the Irish wouldn't have potatoes and the middle east wouldn't cook rice but still I wonder some times.