Because Alice is a good manager who cares about their reports and is otherwise supporting them, advocating for them, pushing for changes to team culture, etc.?
The fact that they can't control this one thing does not mean that they should just abandon the whole company. If Alice finds a company where they can get similar compensation for similar workload without the forced bucketing, perhaps that's a good idea for their mental health, but Alice leaving is a large negative for the team.
>When back pressure and fighting for your reports does not work, what do you do then?
Continue fighting the battles you can win. Do your job and do it well. Changing jobs is hard, stressful, unavailable to many people for a variety of reasons, and not guaranteed to improve things. Particularly once you start becoming senior and in management.
If I left a job every time I was faced with a bad situation I would never built up the soft skills or connections to be any good at any connection. Particularly as a first-level manager, where 80% of your job is delivering messages you had no say in but have to own anyway.
The fact that they can't control this one thing does not mean that they should just abandon the whole company. If Alice finds a company where they can get similar compensation for similar workload without the forced bucketing, perhaps that's a good idea for their mental health, but Alice leaving is a large negative for the team.