Celebrity has chosen to maintain a "traditional" approach to cruising. Traditional assigned dining times, while many are offering more alternatives, and traditional attire. Celebrity also still has designated semi formal/smart casual nights, which the others have abandoned.
I do agree that the cruise lines would remove all the discussions and arguements about attire if they enforced their own rules.
Though I personally haven't seen it, I am told on Celebrity have been turned away from the main dining room for inappropriate dress, and directed to the more casual Lido Deck dining area.