I am not sure how the screening proccess works for luggage sent down from check-in before it reaches baggage sortation but i'm it does get screened or at least theres an operative watching the x-ray images of all luggage and anything remotley suspicious then gets flagged up and thrown out where it undergoes a screening similar to the unnacompanied luggage.
If they still can't see what the item is then they will call the passenger down so that they take a look in their bag.
There are non common sense issues with security i agree such as Airside Personel may not carry liquids through security even though that person maybe in charge of ensuring the safety af the aircraft or maintaining the aircraft and know that if they wish to do something meliciouse the it's easy enough to do so with a tin of soup/yougart etc or not.
Why should staff have to pay inflated fees for beverages airside because DFT would not allow them to take a drink to work. I wonder if a case would be viable of a staff member taking the DFT to court over having skin cancer after repeated exposure the sun as he is unable to take his sun tan oceon through security. (i know thats a bit daft but there could be other examples too)