They are. THere are tickets on sale now for those who want to purchase them early, but for now they're withholding a fair amount of tickets for match day.
If you read a couple of the other threads on the topic, there are people that are under the impression that the game is sold out, or bought shit seats because they thought that's all that was left.
Surely if a seat is unsold, then it would be available to be purchased. Why wouldn't it be?