Like shooting at anything one instances with they are both part of Open.
As to why are they in Open - it's the only populated game mode available to players who have not yet found a PG that suits their play-srtyle preference. It is also the only game mode with an unlimited population. The maximum membership of a Private Group is also miniscule when compared to the number of players, takes a long time to fill, and not all members play at the same time (if they are still playing the game at all - noting how long it takes to grow membership).