Fair question. There are lots of things which happen which are technically illegal which no one cares enough to prosecute. That doesn't mean those things are safe or a good idea. A person I know told me the story of his company deciding "Well if the government doesn't feel like prosecuting our employee for doing X, and they know about it, then probably it's fine" and then the dude got federally indicted a really hilariously short time later. These things do happen; that's why I bring up Aaron Swartz and the people the RIAA sued.

Reddit is a different company from Twitter with different leadership. Twitter seems to be going in the direction of "interoperate with the fediverse and try to steer it" while reddit is still stuck in the mode of "keep stubbornly fucking up and search for people to blame and punish." (Twitter's doing that also, just as part of the fediverse now while they're doing it.) I think the risk of legal action from reddit is a lot more realistic.

It'd be different if I thought that mirroring reddit was a perfectly reasonable thing to do, and they were going to try to prosecute people but that was wrong and so distributing reddit's contents to unrelated people's instances was involving them in something noble. My personal feeling is I don't think that though. I feel like if reddit wants to keep their content off the fediverse they should be allowed to do so, and so you're putting other instance operators at genuine legal risk to accomplish something which shouldn't be done in the first place. Just my personal moral / ethical opinion on it, which colors the way I look at the legal side.