Originally Posted by
Recreate.me
From this i would say that a lot of the people making there times public are people who think they are quick (some probably are) and the slow people probably don't want to be last so they keep it private.
I think you are over-thinking this. Strava defaults to public view. So unless you manually go in and make a ride/segment private it's there for anybody to see. I think it's like Facebook, many people don't realize their photos are there for anybody with a Facebook account to see unless you make them private (which very few people do.)
As far as serious vs. non-serious I see lots of amateur racers at the top of the segment leader boards here in Los Angeles.