If so, than the GUI should not say "enabled", and certainly it should not say next to it: "Status: Running"
In reality something is clearly going on even if there is no user.
Connecting to FTP server is giving different errors depending on the state of the switch:
Enabled (no user assigned): "Connection closed without indication. The connection attempt was rejected."
From time to time also "Error 500"
Disabled: "Connection refused (Connection refused). The connection attempt was rejected."
If the error message is consistently different, than the state of the server must be different - maybe not ftpd itself, but the firewall rules change?
EDIT: correct, when FTP server is set to Enabled, even without any users, port 21 is open, whereas with server set to disabled it is closed as it should be.