If you play alone of course you cannot expect to have the benefits of playing together with others (Wings). Open play has other Wings too which increase the overall risks of players.
Yes, by playing Solo we lose out on the ability to form Wings. And by playing Open, you lose out on the safety from PvP 'muggings'
The only players who should be fined is those who frequently switch between modes to grind with maximum safety and then switch back to Open Mode like nothing happened and buy the top ships and modules with the credits that they made in the safest solo mode.
I agree, I don't like that either. I'm just not sure what Frontier can do about it without artificially tinkering too much with people's money/ships etc.
As said above, it's not really a competition. If you want a combat ship it doesn't take too long to grind your way to a Vulture in any mode, and then what? "I have a Vulture and 20m CR, he's been playing Solo and has a Vulture and 50m!!" I wouldn't care. Personally I wouldn't mind if someone were making a lot more money than me in another game mode, it doesn't really gain them anything. If anything, the players who do that are going to be over-equipped and under-skilled, up against equally-equipped and more skilful opponents.