Why is this still going on? If you read almost anything reasonably intelligent about snowboard sizing these days, it will state that weight is the key (or even the exclusive) factor. Why haven't all shop clerks gotten the message? What kind of bubble could they possibly be working in?
Laziness. It's much easier and less embarrassing than simply asking for someone's weight and then figuring it out from there.
Plus, if you're asking them for which size you're probably still pretty new and you won't be able to tell the difference as much.