On some Japanese BBSes, spammers tend to use non-Japanese IPs or data center IPs. A good chunk of the spam goes away by blocking non-Japan IPs (easy to do with BGP data) and disallowing data center IPs (these often host VPNs, scrapers, etc.) from posting.

Posting from overseas thus costs money or is not possible. The trade-off is 1-100 extra users or significantly reduced spam for little effort. It's not surprising that most website operators choose the latter.

I also know of a file uploader that recently had to block overseas IPs due to such IPs repeatedly uploading illegal content. This is an example of a few bad actors ruining things for everyone.

Blocking IP ranges by country or ISP is pretty much always going to have to exist as long as certain countries and ISPs turn a blind eye to abuse.

Even with as poor a solution as IP blocks are, it's the best we have and alternatives seem worse.

Anyway, it's a tradeoff between dealing with bad actors effectively and not impacting common users. There's a lot more bad actors than common users running into those sorts of IP bans though.