If this is why people want their instances to do it that’s mainly because lemmy hasn’t give appropriate tools at the user level to block entire instances unlike Kbin or Mastodon. (For now you can kind of block seeing them with apps like connect for lemmy, but someone from that instance would still be able to see and repply to your comments, or send you DMs)
As to why a user would want to block an instance and not just particular communities, there are many use cases, for example I didn’t want to bñur NSFW images but I didn’t want to see my frontpage filled with porn, so instead of blocking 10000 communities I just blocked 2 nsfw instances. There is also the culture fostered inside some communities that can lead them to become filled with alt right people or openly facistic rcho chamber which often harrass people, especially minorities.
I agree completely this request is dumb, but this doesn’t look like the community to have this discussion.