Maybe, just maybe, it's because people don't want to be forced to engage in PvP? You're a griefer, we get it. You like flying around with your friends shooting up lonesome harmless clean players. Tough luck buddy, you'll have to work out your real life inferiority complexes in some other game.
Thanks for adding Solo play Frontier. We're enjoying it a lot! If you want us to play in the open world, add a nice PvP checkbox.
Pretty much the case..
It works pretty basic when you think about it logicaly..
1 open mode server. That's it.
The Group mode just keeps anything but those in the group from SEEING the other players, but don't change anything else.
The Solo mode is the same thing, sane Open mode server, just the database is t not to show the other players...
It's just a standard database search of what data is displayed to the player.. the 'mode' just changed the displayed search from
0 = open mode (display all)
1 = Group (Display only users on {group} list)
2 = Solo (Don't display other users)
In those options the world continues, the effects still happen, but if you want to be part of that effect you should play Open or your input might be a but small.