Because it can be both depending on the command line flags sent to it.
Also, because it's so easy to setup, most DevOps/SREs/Ops just chuck into production without reading about which flags to set because we are not informed it's a requirement until 11th hour.
You're making inigyou & OP's point for them. Redis is a great technology, but its design (supporting both persistence and non-persistence modes) makes it much easier to misuse and much more likely to be misused compared to Postgres and Memcached. That's a design issue, not just an internal documentation issue.