Because the person in the salon or restaurant shouldn't be forced to talk to a bloody computer instead of a human which shows some emotions.
What if you literally can't talk like stephen hawking? Or have crippling social anxiety? Or you don't speak the local language very well? Is it still "unethical" for those scenarios?
It wouldn't be "unethical" if it is plain that there is a machine involved. Google could e.g. say something like 'hi, google assistant here' and clarify the situation (and give a human person the choice to proceed). What do you think?
What if, as the demonstration showed, English is not your first language and you struggle on the phone?
What about foreigners on holiday, could their ask for a reservation in one language to Google Assistant and have it handled properly?
Intonation is not the only way of showing emotion. (Regardless of whether I agree with it being unethical, a robot voice != a talking robot.)
Is it unethical for me to make a reservation using a completely deadpan, monotone voice?
The deadpan voice would fall back on you, a computer won't care. But I agree with what I suppose you want to point out, that ethics is not the relevant criterium.
The ethics of it is the interesting part though, if there's a reason we should not do something it should be discussed.
For me, "ethics" is a too strong word for this discussion.
What is the right word, to me ethics determines if something is good or bad, which I think is what you were trying to say.
So it's ok to force someone to talk to a human, but not a computer?
How much emotion should I be showing when making an appointment. I could see your point for familiar shops like a frequented salon or something.