Yeah, that was really annoying. I hated and still hate the bots and the advertisement and the incessant arguing but that ban for making a comment on another sub was absolutely infuriating. And it was always a shadowban, at least in my experience, they never notified you, you only came to realize it when in the middle of a conversation people just stop conversing. Fuck Reddit.
Had that happen to me a couple times, though it was a smaller sub, and the mods knew I was a regular. I posted, forgetting I was in one of those subs, to tell a right-wing troll what I really thought of him, and the autoban went down.
Though in my case, it wasn't a big deal - I appealed, worded my appeal in a friendly way, and I was reinstated in 5 min.
I think in this case, the autoban was put in place simply because of the nature of the sub - it was a leftish political sub that got brigaded and trolled regularly by Reddit's resident chuds.
I get it in the case of a political sub. What I don't get is getting banned by a sub that has absolutely fuck all to do with politics just because of the political views of the moderator(s). It thankfully never happened to me personally, but I find the concept very fucking atrocious.