This doesn't make sense to me either. I don't like the chairs at 90 degrees. You'd have to twist your neck or turn in your chair to look at each other, wouldn't you?
Placement of chairs depends on the size and shape of the room, where the tables are, how many people are in there, etc. I think what's important is to be able to see each other, respect personal space (not too close or too far away), be comfortable, etc.
I have heard that sitting behind a desk with the interviewee on the other side is distancing. It makes the interviewer seem more authoritative. That could be either desirable or not.
Most people don't like to have their backs to the door.
Usually, you want to be able to maintain an open, comfortable body position. Swivel chairs without arms aren't very comfortable in my opinion. They're awkward to get in and out of without worry about scooting or losing your balance.
Other than that....I agree. Who cares?