Let's be clear, the premise of this decisions is false, and the real justification lies in the ideology of the mods.
Throughout this thread you can see users like /u/SavanaJeff, /u/TheBeerCannon and others praise this move because it will hide the discussion that comes with articles on immigration.
From previous posts we know that the mods share these kinds of opinions and we can infer that this move, hidden in the language of neutrality and based on the false premise that there are too many stories about immigration, is based on their ideology.
For example, the "controversial" tag that appeared (and then failed) shows that they dislike the comments that tend to come when a story about immigration is posted. That failed, so now they say that too many stories are posted.
Yet if you look in this thread you will find evidence against that, from opposing perspectives, to word analyses. Immigration is an important issue right now, so it's not unreasonable to have on average 2-3 stories on immigration on the front page. Sometimes it will be less, sometimes it will be more.
So if there aren't really too many stories what is going on?
Well it is one of two options. Either the mods are actually using this as a secret (but obvious) move to limit the "controversial comments" they so abhor, and in their private discussions this is the goal, or it is something much more innocent.
Because the mods, we know, are troubled by these comments, whenever they see the comments, they make a stronger cognitive impression on the mods than other articles. So to the mods it does feel like there are too many articles on this issue.
The problem is now, important and interesting news (as demonstrated by their upvoting of these stories in the sub) is being placed somewhere people who are interested are less likely to see them. Stories, whether they are positive or negative or neutral are being made more difficult for us to see. This means we are less informed about what is happening, and things we are interested in, regardless of our political positions, are being sacrificed because mods don't like the "controversial comments" surrounding them.
So mods, if it is the former, that you simply don't like the comments, then don't be cowards. Just say it. That's fine. If it is the later though, then repeal it and stop hurting everyone interested in this issue because of your biases.