Personally, I prefer individual users being empowered to easily block instances over instances blocking stuff "for" the users in most cases. Issues:
Users from other instances can still require mod actions. Moderation time is limited. Defederating from more problematic instances can be necessary if they cause more trouble than can be easily dealt with.
It is important for instance owners to achieve a coherent "front page" which includes the wider fediverse. I’m unsure if it is possible to ban individual instances from the frontpage while still allowing users to specifically visit them as they want.
Some instances are legally ambiguous or even contain content fully illegal in some countries.
Note that if you click on an instance, it will show you the various admin reasons for why people defederated.
The one I saw someone asking to be removed (exploding heads) seemed to be more normal discussion with a big extra dose of edgy humor magazines and swear words. This includes various slurs and straight up racism. This very much falls into the category of "I don’t want those here, but I’d prefer if users can still visit them" for me.
Specifically, admins and community moderators of that instance were the problem. This seems like it would quickly fall into the "unfeasible to moderate on a case-by-case basis" category. Therefore, the nuclear option of defederation may be necessary.