There's no ethics as far as I'm concerned.
If my company wants me to travel on Sunday night in order to work from 9am on the Monday morning in a different location I do not get paid that extra time away from my home. Nor do I get recompensed for the time away from my family, or for the facilities I have at home that I have paid for.
In return my company lets me keep airmiles and hotel points - it's considered a fair deal.
I suspect the only people who consider it unethical are those who don't travel for work and in fact are merely jealous.