The Oakley's meant for cycling have a straight stem, whatever that means. So their cycling ones are actually different - it is not just a label. If I recall when I was shopping for glasses my Oakleys or maybe it was the other pair I was looking at were available with the straight stem for cycling but I wasn't into bike yet at that point and I just went with the normal version. I got the Racing Jacket and love em. Aside from the super cheap bin of terrible looking frames at the eye doctor, the Oakleys were really no more expensive than most of the other frames (most of them being >$200 and much more than the Oakleys actually). So the tables are turned when we are talking prescription glasses. And my health insurance paid a good chunk of them since they are prescription, and my tax free health savings account paid the rest.

To the OP... "necessary" is probably a poor choice of words. Probably 90% of cyclists don't wear cycling sunglasses so how could they be necessary?