The idea isn't that political discussions are unwelcome, or worthless... Rather, it's that people (and comments) are much more emotional, knee-jerk, irrational, etc. - there is barely any actual discussion going on, it's mostly name-calling, strawman arguments or moral/value discussion that basically amount to "I'm good; you're evil" (e.g. both sides of pro-life vs pro-choice debate).

AFAIK noone has figured out how to have a substantial political discussion, at scale. Until that problem is solved, it makes sense to just tune it out a bit.