No, you can buy it separately but with Season Pass you gain access to all of the DLC that are going to be published in the future. It's like an all-in-one pack and it costs less than if you're going to buy each of the DLC separately.
Think about it. Would it be good PR in the alt. universe if Washington had taken the name "King George", right after the rejection of a King George? Slight bit of brilliant thinking on Ubisoft's part, in my opinion.