If It Tastes Bad, It Must Be Good for You/Analysis

There are two axes for food, or technically almost everything:

If something is in the lower left quadrant, you won't want to eat it and no one will want you to eat it. If something is on the right side, you'll want to eat it whether or not someone else wants you to eat it, so no one is going to come out with the "it's good for you" justification/exhortation. Only in the upper left section -- yucky but healthy -- are you likely to hear that. So it makes perfect sense that if it tastes bad, and a parent (say) wants you to eat it, it's good for you.

It should also be noted that taste is subjective, while healthfulness is not. So if a person doesn't like something, they don't like it, and arguing with them is unlikely to change that. "It's good for you" is much easier to demonstrate.

Evolution
There is actually a scientific reason that "healthy" foods are less flavorful in general. Back during our hunter-gatherer days, food was much more scarce than it is today, and so food that was high in fat and sugar (and thus loaded with energy) would be more sought out and thus more pleasing to the taste buds. Salt has been prized for centuries (remember the Bible quote "you are the salt of the earth"?), as it can be used to cure and preserve meat for long journeys, not to mention its role as an important electrolyte. Unfortunately, like the appendix, this taste for sweet and fatty foods still lives on in our genetic structure. Couple that with our less active lifestyle brought on by technological advances in agriculture, we end up consuming more energy and burning less. Thus, foods lower in fat, salt, sugar and calories are dubbed more "healthy" than our more decadent choices.