Blizzard use discourse for their forums. There are trust levels, these trust levels don’t transfer between characters so I think some just post on their forum characters which are/were their mains in Retail and it’s what they have the most perks on. Or they take part in both forums and don’t want to keep switching.
Pachimari wrote a great guide on it all
There are of course a minority who like to go in either General Discussion forum and troll but I think the majority are genuine.
I don’t get the choice, I am asked to consistently post on one character 