Turalyon is human; humans are in the Alliance. What did you expect? What else was he going to do, hang out in Lordaeron with the Forsaken? Sylvanas is on record that it's no place for the living; she was going to kill her own sister before she let her move in. Pretty sure Turalyon isn't interested in dying and being rezzed by Sylvanas. If anything, I'd say Alleria was pre-disposed to choose Alliance because of him. The reaction of the Sunwell to her presence sealed the deal.
Edit: There's also the fact that Turalyon committed to the Lightforged thousands of years before we found him again.