And with that you're basically admitting it was a missed call on that fumble. I'm not sure you're seeing the disconnect in your case on the game-time result (no personal foul penalty) and the post-game penalty levy stance you think should happen.
If it was incidental, then yes I agree it would be OK to not call a foul. But if it isn't, it absolutely implies that the Shazier spearing wasn't incidental, which is where the refs made the mistake on the judgment call.
Judgment calls aren't easy, but it's very much bullshit when refs decide to be lenient on one side of the ball (Shazier), but fair/harsh on the other (Burfict's hit on Antonio) on such things. Consistency is key and that's why we're even having this discussion. The Burfict hit looked harder than the Shazier hit, but both knocked out players with severe head trauma: Bernard, and Brown respectively.