Yeah, I've read a little about outlaw motorcycle gangs and it seems they are most likely to open carry a fixed blade.
The guy I saw at Circle K wasn't patched, so I don't know if he's a 1%er or not.
Regardless, it is a rare sight.
There was another recent incident in which a Hell's Angel from my working area of San Diego was shot by a federal officer at the Sturgis bike rally. The Angel picked the fight with the fed, but didn't have any weapons that I recall reading about. Their penchant to carry weapons can't help them in a situation like this, even if it is legal in South Dakota.
As far as concealed versus open carry, that goes back to the days when it was considered sneaky and suspicious to hide a weapon instead of openly displaying your armed status. Therefore, open weapons carry is usually less restrictive than concealed. Modern sensibilities makes this seem odd to us, but in a rougher time, it may have made sense.