Keep in mind that some of them are using libwine ;)
I've bought some games (older games from years ago) on gog.com for os x, they were always just running wine
It doesn't really matter if they are though. It means we get "official support" - if something is broken, they are able (and I daresay, maybe even willing !) to look into it.
And DOS-Box. GOG.com Masters of Orion 2 and many older games work great on it.
HumbleBundle games for linux have a history of being... suboptimal. Maybe they've improved over the past year, but HB doesn't really have a good method for fixing bugs in releases (unlike Steam).