I absolutely agree with what you have said that felons should never be trusted with a weapon!
Now for the problem... If a felon is caught with a weapon being used in conjunction of a crime, the weapons charge is dropped.
The courts have ruled that a weapon is an acceptable tool of the criminal trade... Go figure?
That last sentence there is hyperbole, and that's being generous.
The weapons charge is not always dropped. Sometimes that's the only charge they can prove. The possession charge itself might be dropped every time under your described circumstances. However, the possession of a gun during the comission of a crime is a seperate charge and is used all the time.
A felon being involved in criminal activity involves a more punitive sentence than mere illegal gun possession. Given that sentences usually run concurrently anyway, unless there is some compelling reason to run them consecutively (and burden the rest of us with the associated costs,) why should Law Enforcement or prosecutors spend time and resources on a charge that, if successfully prosecuted, would not change the felon's sentence in the least?