I believe it goes both ways.
I found most (not all) of the guys working in the part stores around here have no clue about anything they are selling and if they can't find it in the computer, they just say they can't get it even if they do stock it. Luckily an O' Reilly's opened up down the street and they know what they're doing.
I agree though that people need to know what they are working on before they start asking for parts, my uncle worked at the local Advanced about 20 years ago and he had a guy walk in and asked for some spark plugs for a Buick deuce & a quarter, when he asked what year, the guy didn't know, when he asked what engine, the guy didn't know, he said he needed more info, the guy said the car is blue.
That's exactly it, the manager at our local Autozome knew cars and even had a few real nice ones himself, but when I asked him about a part time job, he told me they would never hire me unless I acted like I knew nothing about cars because they didn't want people with knowledge behind the counter. From what he said, they figured that a knowledged person would not sell someone parts they knew they wouldn't need but someone who knew nothing of cars would sell them whatever they ask for.