A user who has a tcsh or csh as his login session can only start a KDE session from kdm. 

I found the reason for this in /usr/X11R6/lib/X11/xdm/sys.xsession in this script in line 54 this happens:

case "$SHELL" in
*/csh|*/tcsh)
    while read env ; do
        var="${env%%=*}" ; val="${env#*=}"
        test "$var" = "umask"  && umask $val
        test "$var" = "$val"   && continue
        eval export $var="\$val"
    done < <($SHELL -c 'test -r /etc/csh.login && source /etc/csh.login;
                        test -r $HOME/.login   && source $HOME/.login  ;
                        printenv; echo -n umask=; umask')
    ;;
esac

Actually $SHELL -c start a new tcsh or csh depending on the users shell. This finally will source /etc/profile.d/profile.csh and in this script the variable WINDOWMANAGER is set a new to the default windowmanager which usually is kde. Becvause of the code from above in sys.xsession the new setting for WINDOWMANGER from profile.csh is afterwards also exported to sys.xsession and so the script always starts a kde session.

The code in /etc/profile.d/profile.csh the resets the WINDOWMANGER variable (which should not happen if called from sys.xsession) is the following:

foreach val ($path /usr/X11R6/bin /opt/gnome/bin /usr/openwin/bin)
    if ( ${val:q} =~ *.* ) continue
    set val=${val:q}/${default_wm:q}
    if ( ! -x ${val:q} ) continue
    setenv WINDOWMANAGER ${val:q}   # <-------- it happens here
    break
end