I am a little puzzled.
SID heights restrictions are normally programmed into the FMS automatically when the SID is selected. Departures are normally flown in VNAV. Fortunately I have never tried it but if you were to make that same error say out of Heathrow and put FL310 in the alt sel the aircraft would level off automatically at the SID altitude of 6000. Even if hand flying the flight directors would level you at 6000. So my question is did they push the alt sel to clear the initial level off altitude or do IAH SIDS not have level off altitudes programmed into the FMS? If the latter is the case then why not as it would prevent this exact scenario.