When UEFA threatened England and Russia with being thrown out of the tournament if further trouble occurred on the streets, a number of people wondered how this would be possible without affecting things like third place qualification, forfeit scorelines, scrapped matches etc.
However, if UEFA throw both countries out together, there will be no third place team in group B and the four qualifiers would be found from the five remaining groups.
Also, if UEFA don't make the decision until after the group stage had finished - perhaps using the fabrication that the French authorities are too stretched to cope with the ongoing disorder - then no matches would have to be scrapped and the second round line-up would be unaffected.
Additionally, if UEFA are reluctant to punish the Russians for political reasons, the decision would be made more palatable to both parties if Russia didn't make it through the group but England did, because England would sustain the real punishment and not Russia - in the same way that UEFA gave Russia a suspended sanction that just happened to expire before the tournament began.
This is all coming off the top of my head, but if UEFA are really serious about making the definitive statement by ejecting sides from the tournament, this method would seem to enable them to do it without upsetting the apple cart of the tournament itself - and, perhaps as importantly, the Russians.