You gotta keep in mind that Blood Elves themselves left the alliance because they were getting treated like crap and then they renamed themselves the Blood elves, while high elves still stay loyal to the Alliance.
Also, the alliance changed quite a bit, with now Varian in control and more then just humans and high elves, the other races tend to recruit other races like the Gilnean Worgen.
The reason why worgen join the Alliance makes perfect sense, you either have to be ignorant or stupid to not accept that.