To be honest though, just because you can get by without knowing how hash maps are implemented, doesn't mean it isn't useful to know - and if another candidate does know, they would sooner hire them.
Further, they may be looking for factors that correlates with other, hard-to-measure factors. Even such as reading a "common interview questions" book - it correlates with focused pragmatism...
I'm not saying it's not useful to know or that having that knowledge might make someone a better candidate, but it seems like that alone shouldn't be reason enough to reject someone.
And I disagree that looking up the answers to common interview questions is focused pragmatism. I don't bother, even with the knowledge that it would help me get a job. I spend my time working on side projects or learning practical things. I think I will never in my life have implement a hash map, and that knowing it's all O(1) operations is sufficient. But also I guess I don't want to work for a company that is interested in my ability to jump through hoops. (shrug)
Further, they may be looking for factors that correlates with other, hard-to-measure factors. Even such as reading a "common interview questions" book - it correlates with focused pragmatism...