In many parts of the world (Asia, Europe, the Middle East), people eat testicles. I don't know how healthy they are, but they are supposed to be tasty, and in some cultures considered an aphrodisiac. They are often called animelles or lamb fries (the latter referring to sheep testicles). Yes, you can get these in the USA too. I personally think one reason why most Americans never hear of such things is because most Americans can barely say the word "penis" without reverting to the mentality of a 13 year old and giggling like a prepubescent teenager. Historically speaking, when people didn't eat mass-produced, highly processed shit (ie most of the American meat industry) people would actually consume nearly the entire animal. Parts such as cow tongue, cow brain, etc were considered delicacy.
I think that the "tastes like balls" idiom is meant to evoke an image of licking a scrotum attached to a live human male, not of consuming cooked and seasoned testes. So, I don't think that this idiom is a fair example of American narrow-mindedness with respect to offal.