Yes, it does happen all the time, and players get carded for it too. He made a dangerous play in the ruck, contrary to law 9.20. The TMO deemed the offense to be sufficient to warrant a yellow card. The judicial committee found that he made contact with the neck area of Marx, which interestingly would have put him afoul of Law 9.20(b).
The yellow card was entirely appropriate. Not exactly sure what your overall point is, so do with that information what you will.
If by 'neck area' they mean 'shoulder' then sure. But there is no evidence that Barrett hit the neck on the Stan footage. There may be some better footage they have.
It doesn't matter - refs get calls wrong and I'm of the firm belief that teams have to play at a level to take refs' calls out of it as much as possible. For me the problem is that Marx is clearly on the wrong side and given that cleaning him out puts you at risk of being carded, how do you get rid of him?
The very first penalty France won on the weekend was for Papali'i holding on. The problem was that the French No.1 is on the wrong side of the breakdown after making the tackle and Tupou Vaa'i trips over him trying to get to the hooker making the turnover.
Instead of the tackler being called for not rolling away, Papali'i is called for not releasing the ball.
I'm actually quite interested to see if the All Blacks (or any team) come up with a way to combat this tactic.