The security screeners are only doing what they are told to do. Its the government who implement the rules to cover their backsides in the case of a terrorist act.
The reason why you have the same racial groups working for the security company is that the company encourages staff to bring family members to the recruitment seminars to promote harmony and engagement with the staff.
Whoever the screeners are they are also subject to police and ASIC checks before they are let loose preventing the killer tuna on an aircraft.