Hi,

Im a vampire, stage 2 and I just walked inside the fort Dawnguard without anything on my head, my eyes are glowing and still I could talk to Isran and he gave me the first mission.

WTF

Convinience, maybe? Otherwise, if you are to join the Dawnguard after you brought Serana back, then you have to go get cured.

To let it be fair to all players. You'll eventually make your choice.

Because joining the Dawnguard is required to begin the DLC and to meet Harkon. Think about it: if you're a vampire and want to side with vampires, you'd have to cure yourself to talk to Isran, then do one quest, join Harkon and become a vampire again. Quite pointless.

It's poor writing, sure, but what can we do. You'll be forced to make a choice soon enough, so don't worry.