> why doesn't mgetty kill the user's shell when the user just hangs up
> the modem, without doing 'logout'? [Mod: or the line simply crashes]

How should mgetty kill a user's shell? While the user is logged in, mgetty
is *not running*.

The kernel will signal the shell via the SIGHUP signal when the DCD line
on the modem drops. Then the shell will exit, and init will re-start
mgetty. Unfortunately, the BASH shell is very broken in various versions
and will ignore the SIGHUP signal, so that could be one reason why it
isn't exiting. 

Another reason could be an improper modem setup (AT&C0), make sure
that the modem raises and lowers the DCD line properly (check with the
modem manual) and that your serial cable is OK (swap it with another
one for testing).