Gregory's Server@grishka @organicmaps Agreed, but I assume this is similar to YouTube requiring you log in to see restricted videos so they can verify your age. (I haven't confirmed this though, and if the app doesn't require a login with age verification, this argument falls flat)
Most likely Reddit is cutting costs on moderators as well which is why smaller communities are unmoderated now. (Which makes a good point for your in-app ads argument)
Bart, so I tested it. This only pops up when you aren't logged in. So they're still lying to you by saying "please use the app" instead of "please log in". But even then, you can still view that same damn "unreviewed" subreddit anonymously if you open old.reddit.com, so the account requirement is, of course, entirely artificial ¯\_(ツ)_/¯
YouTube is at least 100% consistent about it.