Thats not what bothers me. What bothers me is that a vampire can waltz in there and consider joining without being attacked. You'd think they would, of all people, know you're a vampire. Why was it written this way? Why force us to even work for them? As a vampire? Or an evil character?
I think it just builds on the background story that the Dawnguard
are initially clueless and so small a force that a vampire
could waltz right in. Isran even says as much when you talk to him.
But I think that that's why it is written as such. Just to place the figures in the proper light for the beginning of the story.