Sorry I mistook the screenshot for somewhere unreachable. Generally if a person can be reached by melee, it's allowed.
So in this case a melee player could run around and kill him. Compared to castle assault people staying on the castle walls end up being barricaded from a vantage point with ammo, so wouldn't be allowed.
Since the player is reachable, it's considered tactical: they're otherwise distracting attacking players from pressing the objective, which in itself is benefitting the defending team
Although it is a bit of a dirty tactic. But then again, it's an archer; what do you expect