1. Actual ethics have virtually no place in D&D and probably not in violent fantasy to begin with. They get in the way of the game -- the fun bits -- and also the descriptions of the world(s) are too unstable to really yield meaningful discussion.
For example, are Orcs inherently evil savage beasts hellbent on destruction and corruption (Tolkien), inherently savage beasts hellbent on destruction due to their natures and for the shits and giggles of it (Warhammer 40k), or just bigger greener nastier people (occasionally D&D). All of these lead to different ethical considerations.
And, it's hard to play a mass murderer (any D&D character) while considering actual ethics.
2. Demons, et al. are, however, inherently evil. They spend every minute of every day either doing or plotting evil. So, appropriating them for even a few minutes on end is doing the universe a favor, assuming you're a good guy. An evil one wouldn't mind that summoning was slavery anyway. That's 5 minutes less -- perhaps more if there's recuperation time -- of evil being produced. Even if the demon dies, that's a net benefit as well. I mean, would your character have had any qualms about killing that demon outright in the first place?
Conversely, celestial beings want to do good with every fiber of their being. They have no greater purpose. So, if you're a good guy -- e.g., a Lawful Good Cleric -- then presumably you are summoning the celestial creature to do good things. Therefore, it's happy to help. QED.
You could make a similar argument with Animals and Elementals, probably, as they are committed to helping out the Druid with the Druid's causes -- whether for good or for ill.