Just because they didn't find any Night Elves after they were reported doesn't mean they weren't there. Night Elves are masters of stealth after all. The reports were valid.
We go there to investigate another report about a night elf sighting near a sanctum that was suspiciously sabotaged. We actually find the Night Elf, after much searching and we find detailed documents about the Sanctum on their corpse, written in Dwarven.
The evidence is clear; the Alliance tried to sabotage vital Thalassian infrastructure. An act of clear hostility.
In the Ghostlands this is further proven by the fact that they've taken over an island and have sabotaged even more stuff down there.
The Night Elves attack us on sight, the Dwarf attacks once you take him aside. You're wrong.