If we want to see evidence of Yeats's criticism of Britain's drafting of Irishmen to fight in World War One, we need look no further than the third and fourth lines:
Those that I fight I do not hate,
Those that I guard I do not love;
The Irish airman doesn't hate the Germans; after all, they've done nothing to him or his family. Nor does he love the people he's duty-bound to defend. They are British, the very people who've subjected his native Ireland to several centuries of oppression and colonial exploitation. The airman is fighting for a country that isn't his. His country, as he makes clear, is Kiltartan Cross, a crossroads near the Gregory family pile in Galway that Yeats knew so well.
As the airman makes clear, he's not fighting because he was forced to do so or because he was persuaded by politicians—"public men"—or the clamor of cheering crowds. He's only doing so because of a "lonely impulse of delight." In taking to the clouds, the Irish airman is motivated by the satisfaction of his own desires rather than selflessness or patriotism.