I saw the same problem on an SME v5 update3 (which was an upgrade from e-smith 4.1). In testing, I turned on private telnet, and the users were magically removed from the /etc/ftpusers file.
But, the ftp server was turned off for some reason in the process. (Another bug.)
So, I turned the telnet server back off, and the users were still out of the /etc/ftpusers file so they could then login.
Anyway, I'm glad to find simple (but random) work-around to a bug I hope e-smith knows about...