Earlier quoted context omitted.
Then why ask? All job interviews are weird mind games. My assumption when I'm asked why I left my last employer is that they're looking to second-guess my other answers based on what I say, or checking to see if I'm a disagreeable person who is willing to badmouth somebody generous enough to employ me. This is just taking the latter in the opposite direction.
There are plenty of reasons to move employers that doesn't involve saying that company you were working for is unethical. You could say that you've reached your ceiling and are looking for the next step up. That's a fair one. The company you are leaving may not be able to promote you. You're looking to work on new projects and your current company is just maintaining and/or adding features. That's also fair. > checki…
Not convinced you have much space to declare some global idea of "reasonable interviewer" although you're more than welcome to complain about you personally not getting an offer from a company that clearly doesn't share your values. Your words say "badmouth" but that's just a way of hyperbolizing the ability to be vocally self-critical, which is necessary to learn lessons and achieve actual growth as a company.
Most startups have to do this, in my experience. You can't experiment without it. If you can't identify one weakness in your prior employer's model (whether ethically or functionally) or you'll get huffy about talking about that in an interview setting, why would I trust you with the responsibility of being in my organization? I'm going to get to those answers eventually. Why waste your time?
Looking back at history, do you think they called them the "traitorous eight" [1] because they were "reasonable interviewers" who would never "cast aspersions on a previous employer?" No. The history of this industry in the USA is based on vocal critique, competition and paper chasing in the pursuit of out-executing your competition.
If you don't have that culture of critique and open communication, it will be very hard to become #1 in whatever you're trying to do. Maybe that will never matter to you and the companies you join. Okay. If that works for you, great. But I'm a career startup person, so it does matter to me. I would literally /never/ have gotten anywhere without being able to earnestly note what my previous employers did right and wrong, and use those learnings in my next opportunity -- often very vocally.