Yes, they can and they have done so occasionally.

It hasn’t been addressed because so far it has been used relatively sparingly, and because the subreddits in question are often shut down by Reddit rather than handed over to a new head moderator.

Often it happens because the mod(s) refuse to enforce sitewide rules on their subreddit. The removals are usually preceded by encouragement from admin to sharpen up, followed by warnings and milder measures.

Historically, Reddit has been slow to act on that stuff of its own accord, only doing so when users organise to alert advertisers to the content their ads are appearing alongside.

You will see users of these subreddits complain that their communities are being targeted, especially if Reddit has acted against a cluster of related behaviours, but that latter bit does mean that those users tend to seek out other places. I don’t want to compare those fleeing to Lemmy/kbin/mastodon to proponents of political violence or those running foreign disinformation campaigns, but the principle of Reddit pushing a given class of users away from the site remains the same.

I suspect that Reddit has miscalculated here, or is just gunning to manipulate the apparent user metrics ahead of the IPO, but it seems to me that they actively want to shed everyone who is privacy-conscious, tech-literate, etc. They don’t want conversations about old.reddit or 3rd party apps or the fediverse influencing those who are unaware of these things, and they don’t want users or moderators who will organise against Reddit They already have power mods who have been running large numbers of the most popular subreddits for years and some of these may be paid admin; they’d find it easy to get people to replace the head mods of smaller communities, and they probably don’t care about the niche subreddits and will either let those die or trial the use of bot moderators on them.