The case of the Canadian milk tariff actually is pretty simple and when you understand it, you understand that Donald's outrage over it is only another one of his WWE-style stunts. It's about price stability and that's all. There's no malice and no "unfairness" to American producers.
Say Canadian producers need to make 50 cents a gallon to be profitable. They buy enough cows to meet demand and keep the price around 50 cents. If they overproduce and the price goes to 40 cents, they have to find a use for the excess milk, lose money or dump it and maintain the 50-cent price. American farmers, for whatever reason - subsidies, lower interest rates or whatever drove their expansion - greatly overproduced and they're looking for a place to sell their 40 cent/gallon milk and try to salvage the year. Canadian farmers, naturally, don't want the price to go that low, so they put a tariff on it to maintain the price and they tell the Americans to look elsewhere. Donald's expecting Canadian farmers to take the hit for bad planning by American farmers and Canadian farmers are telling them to take off, eh. Simple.
Don't look at me, I just work here.