Goldfish are actually one of the most versatile fish as far as temperature ranges go, and can even live in water even warmer than many tropical fish can handle. Warmer water speeds up their metabolism though. Gold fish are often advised to be kept in cold water because lots of people put them in too small tanks and cold water slows down their growth and makes them produce less waste. Goldfish produce more waste than most fish and warm water compounding that problem is often too much to handle for a novice aquarist. A single goldfish needs 30 gallons and when forced into a smaller tank, warm water makes it so much worse.

In short. Telling people to keep them in cool water is not bad advice. Its just paints an incomplete picture.