Many people, understandably, aren't comfortable with the notion that you "can't say anything negative" about your previous employer. It feels disingenuous to answer the question "why are you leaving?" without indicating, naturally, WHY you are leaving. Leaving a place because it was awful, toxic, or unethical are really good reasons to leave. Everyone understands that and everyone has left a bad job or two (or more) in their career.
The subtle twist here, which even the interviewer may not be cognizant of, is that the question isn't really asking you to list how your current employer is insufficient, malignant, lacking, or somehow blocking your career intentions. The true intent behind question aims, instead, to get at where you are going to rather than what you are running from.
Nobody wants to hire a candidate who seeks to join them simply because the candidate is escaping a sinking ship and the job-offer is like some kind of career life-raft. They want to hire somebody who is choosing to join them because of what they (the candidate) can make of the job. They want someone who has a deeper career-intent than merely to escape somewhere awful.
In fact, even if one's current job is like hell-on-Earth, one should NOT describe that to a prospective employer. Doing so gives them the impression that the candidate is just looking for the first available "life-raft" rather than acting on a carefully-considered career goal. On the other extreme, complaining about small shortcoming is also bad, it gives the impression that candidate can't handle ordinary problems that successful people deal with and overcome routinely.
The way to answer "why are you leaving?" is to describe your upcoming career goal and, hopefully, how the prospective job can meet that goal. It is perfectly fine to say that you've reached an "endpoint" at your current employer and need to change jobs to go further in your career. However, anything that signals to the employer that you're "running from" something rather than "running to" something will raise a red-flag (whether the interviewer consciously realizes that or not).