Well, the point I'm trying to bring up is that basic nutrients (for example, the fats in canola oil or butter) don't easily fall into binary categories like "good" and "bad," "healthy" and "unhealthy." Animal life is a bit more complicated than that- more of a balance game than struggle between Good and Evil.
You might be misinformed even if you consider the cholesterol in butter as making sweet potatoes "unhealthy"- because the fiber you'd get from the potatoes is more likely to bind with your bile salts than the cholesterol in the butter, so you end up passing most of it.
Furthermore, the fats in the butter aid your body's absorption of awesome nutrients like the fat soluble carotenoids in sweet potatoes/yams while boiling removes not only some fat soluble stuff but also a huge portion of the water soluble vitamins.
And believe it or not, butter is more filling calorie per calorie than the carbs in sweet potatoes, so most people will end up eating better balanced portions of potatoes and butter than if they ate potatoes alone!
Like I tried to imply, nutrition is a relatively complicated thing. I had tons of misconceptions when I was trying to understand it by making inferences based on the latest exaggerated fad article I stumbled across in some media outlet. Finding a college nutrition book was one of the best things that ever happened to me.