Snipes evaded taxes to the tune of a very large sum of money. If anything, his sentence was too mild. If someone embezzeled an equal sum from, say, a bank, you can be sure that authorities would throw the book at him or her. Why should the well-heeled and famous be treated more lightly.
Incidentally, the government likes to go after big names in order to send a message to the general public to refrain from tax cheating. This case was a perfect vehicle.