As the ID is never checked how do they know if the person named on the ticket is the one travelling?
There is quite a push from the Feds to reintroduce ID requirements. They are using the "terrorism" flag to get what they want, but is really to combat organized crime ie keeping tabs on known criminals thus getting the industry to do their work for them.
Therefore the bag travelling may not be theirs!
So therefore there is an assumed lower risk that there would be a bag substitution. Why would that happen if it is just Big Joe from the Finks travelling. Also they are working off the old school theory that a bad guy putting something nasty on an aircraft does not want to be on it when it does what it does. These days the bad guy tends to get on board with the bag with something nasty in it. It will have his bag tag on it.
That's what the baggage Xray machines are for. That's why there is 100% bag screening. Then it doesn't matter who's name is on the bag, as long as whatever nasty is in the bag does not get on the aircraft.
In the end reality has to be considered. Someone wanting to do something nasty wants to make an impression. The assessed risk is they won't waste their time (or life) on a smaller aircraft and less people. A line has to be drawn somewhere.