As others have noted, both student and teacher are kinda awful. The student for obvious reasons, but the teachers assignment and grading isn't really best practice, especially if you give a 0 when there's literally any rubric you grade to (and you want that for anything you're grading seriously).
I teach psychology courses at University level and for written parts you really have two acceptable options. Important papers (e.g. final essays worth 20% of total grade) should always have a rubric-- these you can very much give a bad grade to a student who clearly doesn't understand what peer reviewed evidence is because evidence might be a 20pt rubric item and without it, an A drops to a C (and that's assuming everything else is perfect). Having it in writing protects you legally.
The other, much more common assignment like a reaction piece, is mostly to check for participation in reading an assigned reading. There's usually no rubric and it's scored for completion so there isn't any subjective side to it. Quote the Bible or whatever, it's like a participation grade to encourage reading and if you have a bad take, whatever. These are usually 1% of the grade and there's usually one every week to keep up with.
Doing anything in-between opens you up to problems, not to mention it's just unfair to students. If you follow a rubric, you almost never give a zero unless they just straight up cheated.
It is common practice, which is more to the point that it's weird this teacher didn't. However, third rate instructors do exist. I've TAd for some even at a top 25 school who still just vibe-graded their classes. It's really unprofessional, but they were hired for research reasons, not instruction.