why does snowball get blamed for destroying the windmill

Poor Snowball is blamed for everything. Besides the windmill he also takes the blame for breaking eggs, smashing windows, and plotting to attack the farm in collusion with evil farmer humans. The point here is that Napoleon needs to cast Snowball, his perceived nemesis, as their enemy. The Windmill was the hardest thing to build. Ironically it was Snowballs idea and initiative which got the thing going. Napoleon seized the opportunity to blame Snowball for its destruction, and validate his forced exile.