Original: Even in Japan, horseradish dyed green is often substituted for the more expensive wasabi traditionally served with sushi. The Japanese botanical name for horseradish is seiyÅ wasabi, or "Western wasabi".
Simplified: Horseradish is similar to wasabi, so in many places and restaurants, they use horseradish as wasabi, as it is much cheaper than wasabi, which is very expensive.