I wouldn't mind something like, "If you solo the entire raid, you can't get less than 10th-place rewards", but changing the algorithm so that all soloists get 1st place rewards would seriously change how many Stun Guns are given out.
Now, don't get me wrong, I'd love to have more stun guns, but there has to be some resource constraints, and the primary bottleneck in this game is Stun Guns. If they gave all soloists 1st place rewards, that would change the bottleneck significantly and thus affect game balance in important ways.
It would still be "fair" of course, because anyone who puts the right team together can solo the thing, but it would change the game, and EA/CG clearly doesn't want to do that.
For what it's worth, I support the idea that a bottleneck has to exist - if there's no effort to improving your roster, there's no sense of accomplishment when improving your roster. If it wasn't Stun Guns it would be something else.