It's confusing for sure. It's only called that because WMG acquired the independently owned Rykodisc in 2006. WMG wanted an umbrella to house both Ryko and EastWest, which they were revamping from a hip-hop roster to a more "indie" flavored style of music. Hence, the "Independent" Label Group. It also comes from how WMG has changed which labels they distribute for. I know you don't need a lesson on this but a lot of people don't totally get the specifics of how records are distributed. Rykodisc owned its own distro company.
Last year, that company was absorbed by the Alternative Distribution Alliance (of which Sub Pop has a 5% stake and WMG a 95% stake), which distributes releases by what we might consider indie labels - labels like Equal Vision, Epitaph, Eyeball, SideOneDummy, etc. So even though we consider these to be "indie labels" the fact of the matter is, WMG is still mainly responsible for getting those releases pressed and in stores.
They don't provide any of the money to record or run the labels, but they do distribute and make money off doing so. So if you see a CD in a big name store like Best Buy, chances are WMG or UMG had a hand in it, even if it isn't a major label release.