That's the entire point of the fediverse, if you don't like how content is served to you in one instance, you move to another instance. I am not heading over to lemmynsfw.com and complaining about how consuming SFW content is difficult over there either. You can still consume either content on either platform if you want to but having the two main instances, which currently are kbin.social and lemmy.world, focus on SFW content and neglecting NSFW content makes perfect sense.
Many people are going to use kbin in public and then not having NSFW content pop up on accident is super important. And while user customization is always a nice thing to have, I can totally understand that the devs of kbin currently have other priorities. And so taking the more drastic approach is perfectly fine.
Saturdaycat