The problem is that people in the west often can't see the difference between realistic and fictional stuff.
One good example would be the difference between child pornography and lolicon.
In the east, lolicon is just a 2D drawing of a child in a more sexual sense, and it's just considered nerdy and is therefore legal.
Child pornography involves photographs of real children being sexually abused, which is considered a sick thing and is therefore illegal.
However, in the west both ways is considered the same thing, and therefore illegal.
This has caused many owners of western Mastodon instances to blacklist a big number of Japanese Mastodon instances (mostly Pawoo.net), though you should keep in mind that 3 Japanese instances make up for the top 5 biggest instances in user count, status count, and connection count.
Sorted by user count.
Sorted by connection count, which is the only top 5 where not 3 Japanese instances exist, but even 4 of them!
Sorted by connection count.