su Forces Terminal Echo to On

I just discovered an interesting quirk of the su command, at least on FreeBSD: it forces terminal echo to on and you cannot turn it back off. Interestingly, sudo does not have the same behavior. Here is a sample terminal session showing the behavior (input that was not echoed is shown in red):

[14:54:35] myhost:~ $ stty
speed 38400 baud;
lflags: echoe echok echoke echoctl pendin
iflags: -ixany -imaxbel
oflags: -oxtabs
cflags: cs8 -parenb
[14:54:38] myhost:~ $ stty -echo
[14:54:57] myhost:~ $ stty[enter] speed 38400 baud;
lflags: -echo echoe echok echoke echoctl pendin
iflags: -ixany -imaxbel
oflags: -oxtabs
cflags: cs8 -parenb
[14:55:00] myhost:~ $ sudo -s[enter] Password: mypassword[enter]
[14:55:15] myhost:~ # stty[enter] speed 38400 baud;
lflags: -echo echoe echok echoke echoctl pendin
iflags: -ixany -imaxbel
oflags: -oxtabs
cflags: cs8 -parenb
[14:55:20] myhost:~ # exit
[14:55:25] myhost:~ $ su[enter] Password: rootpassword[enter]
myhost# stty
speed 38400 baud;
lflags: echoe echok echoke echoctl pendin
iflags: -ixany -imaxbel
oflags: -oxtabs
cflags: cs8 -parenb
myhost# stty -echo
myhost# stty
speed 38400 baud;
lflags: echoe echok echoke echoctl pendin
iflags: -ixany -imaxbel
oflags: -oxtabs
cflags: cs8 -parenb
myhost#

As you can see, the -echo flag was preserved by the sudo session, but not be the su session. Furthermore, it was not possible to re-disable terminal echo within the su session -- it simply ignored the directive.

I suspect the cause is somewhere in the PAM libraries that su uses to authenticate the user. However, I would have expected sudo to use the same PAM libraries, so I cannot explain the difference in behavior.