Because the YC/HN culture is to experiment to inform future direction. "just for a week" probably means "just for a week, so that we can see if it negatively or positively influences the quality of discussion... and, if the effect is positive, we may implement a similar policy for the long term"
Thus, "just for a week" doesn't appear to have much relevance since the experiment is (presumably) part of a longer term plan to curb hostile commentary.
The long-term plan is to protect the values of this site (intellectual curiosity and thoughtful conversation), or at least, it's our intention to find such a plan. But there's no longer term plan to ban politics. I understand why people would react with that concern, of course, but it really is just an experiment for a week.
That was definitely not clear to me from the post (although it is reassuring).
Also, I've gotten in trouble using that word "experiment" before. It turns out to mean weirdly, wildly different things to different people. But I'm attached to it.