It should be. If the interview tests for what you crammed for, it doesn't test for what you'll remember a month onto the job. That's a sub-optimal strategy for the interviewer.
I occasionally interview people and that wouldn't really bother me. If you can learn something once you'll be able to re-learn it down the road when needed.
I've found myself in that situation helping my kids with their homework. I might not be able to define L'Hôpital's rule off the top of my head, but it only takes a few minutes of reading to reload it into my brain.
Devil's advocate: there exists a corpus of questions solvable with a small number of techniques after a few weeks of decicated study. When someone has the right brain, these questions are the barrier to a 99th percentile income. What does it say about an interviewee that doesn't bother?
That they're unwilling to jump through random arbitrary hoops that are unrelated to their actual work. If a potential employer considers that a negative, what does it say about the employer?
It says they lack the social capital to know what these questions are and how to study them. Is the goal to optimize for people with family and friends already in tech that can pass on the "secret" or to maintain the fig leaf illusion of meritocracy?
