First off, Lycanthropy is available with the game out of the box if you choose to become a werewolf. So, IMO, the tree should have been included in the main game and not as DLC.
DLCs are optional. You should be able to enjoy what you already have at the max, that is, we should have been able to enjoy being a werewolf and be able to spend attribute points to enhance that skill with the main game itself not by spending more money.
With that said, Dawnguard really becomes like a must-have if you want to be a full-fledged lycan in the game. Like something you NEED if you want to feel like you fully completed the main game. It's like saying "Oh we forgot to add a Lycanthropy tree in the main game! So, you pay $60 now then $20 later to get the tree." Plus, Dawnguard was supposed to be about vampires, what's lycan content doing here?
I personally don't like this tactic. People should be able to choose whether to buy a DLC or not without suffering any kind of collateral damage.