Their reasons are very informative regarding Wikipedia's broken culture. A selected review......Users are required to follow proper decorum during discussions and edits. Users may be sanctioned (including blocks) if they make any edits which are judged by an administrator to be uncivil, personal attacks, or assumptions of bad faith.
* "It is redundant with our current civility policy."
Lots of things on these discretionary sanctions are redundant to existing policy. Why? Because Discretionary Sanctions only became a thing because Wikipedia Administrators were not blocking people when they violated a policy under normal conditions. For some reason, as well as the backing of basic policy, in areas of controversy they also feel they need these silly templates, as essentially permission slips to allow them to do what they should be doing normally anyway.
* "Administrators can already block uncivil users for personal attacks, etc."
It's cute how everything else in the civility policy is reduced to a pathetic "etc." This reflects the current culture, where Administrators pretty much don't care about any part of the civility policy except the section on personal attacks. It has essentially become synonymous with the policy. The purpose of this civility restriction being in the American Politics Discretionary Sanctions of course, was to empower an Administrator to block a user for making a single edit that was incivil, but not necessarily a personal attack, in the highly charged atmosphere of these politics disputes. They cannot currently do this under normal conditions, because the policy doesn't allow for it.
* "The template is for sanctions, not reminders. If we want to make it a template for reminders I can think of better policies to remind users about (NPOV for instance)"
It is a sanction. It puts a requirement on a properly notified user to follow proper decorum, and allows them to be blocked for a single uncivil edit. This is not the same as a simple reminder that the civility policy exists. And who cares what he thinks is more fitting, it's a simple matter of fact that the civility policy is equal to the neutrality policy. If he doesn't agree that should be the case, then he needs to resign.
* "It's not clear what constitutes a violation of "proper decorum during discussions and edits"."
And here we arrive at a familiar problem. Not every Administrator can agree on what is proper decorum, so it somehow becomes the only policy on Wikipedia where Administrators are not allowed to use their own personal judgement, even in a situation where they are specifically allowed to use their personal judgement (this is the whole point of Discretionary Sanctions), only deferring to consensus if their judgement is challenged. Needless to say, this aversion to unilateral action does not happen, and indeed is never even considered, for "better" policies like NPOV, even though that too can be highly disputed between individual Administrators.
This small but significant attack on the notion of civility is naturally already on course for landslide suport, because Wikipedians simply don't want there to be any requirement o obligation on them to be civil At all. Essentially, this move will give them carte blanche to be rude to anyone they seem deserves it, in the arena of American Politics articles. You don't need to be a genius to realise who is the intended target in that arena.
For an idea of who regularly gets insulted by Wikipedia Administrators, just peruse the contributions of the first two Administrators who thought this proposal was a brilliant idea - Drmies and Bishonen. A right pair of scumbags, those two. Try reporting them to a noticeboard for a pattern of incivility, you will be amazed at how vicious the response will be.