Why does su command ignore SIGSTOP?

After entering su when I prompted to enter a password: user@debian:~$ su Password: I can't send SIGSTOP (ctrl+Z) from my keyboard (the same terminal) - nothing happens. So the only way to exit is to type some (right or wrong) password. Why can't I suspend su such a way? UPD: It seems that ctrl+Z is queued. Thus when sending ctrl+Z - nothing happens, but then after typing Enter signal arrives and su becomes stopped. Still can't understand such behavior. Is this standard behavior for all Unix-like or just Linux?