They probably didn't know about it.
Eight years ago (six as of the start of the series), they were only just starting to talk about Dragon being a potential huge threat in the next couple of years. We know that from Dalton's flashback. By then we also already know that Jinbei and Doflamingo at least were Warlords, and chances are all the others bar Hancock- does the timeline match up for her to take the space Ace rejected?
Anyway, so the RA weren't a known threat at all really round about the time that Kuma's contemporary Warlords were being recruited. It makes more sense to me that Dragon asked him, as his right hand man, to set himself as a pirate so he could be invited and become a spy in the World Gov. Keep in mind the World Gov's intelligence is far from infallible, as they were totaally unaware of Vergo's true allegiance. It's only after this that Dragon becomes the Most Wanted Man in the World, at which point Kuma has, seemingly for years, been the most loyal of the Warlords.