Waikele, it is not uncommon for airlines to overbook flights hoping there will be no-shows or last minute cancellations to avoid boarding denials.  If a flight is still overbook at boarding time, gate agents first ask for volunteers to give up their seats for certain compensation.  If that does not work and the flight is still overbooked, airlines are permitted by federal regulations to select the passengers who will not be allowed to board.   All of this should be done before anyone boards the plane.   This is all in accordance with Federal regulations.  The problem with the Dao case is that passengers were boarded and then asked to give up their seats.  That definitely was United's fault and the cause of the Dao incident.