PostgreSQL logical replication slave - shouldn't it be read-only and available only for subscription updates?
With PostgreSQL logical replication, the table on the standby server (where the subscription is) is available for data modifications. This is different from physical replication, where the standby is a physical copy of the primary and cannot be modified directly. Perhaps your confusion originates there.

While you can modify the table on the standby, it is not necessarily a good idea to do that, because it can lead to replication conflicts. But it is in your own responsibility to not perform any such operations.