Well, many women's teams withdrew. So can't host replace one of them? 10 teams could participate but only 8 arrived and Russia joined them. If Russia didn't participate we could also say that two winning teams can't go to Olympics -- they didn't prove their power comparing with two absent teams!
It's a common rule that a host nation can participate in the events, and it doesn't matter what this event is. Actually there should be reasons WHY hosts don't participate, like no true rugby team or ineligeble athletes.
Does really hosting of two beach volleyball events in 4 years mean to you that Russia controls FIVB? Oh, it's a paranoia actually.