It’s a part of standard sentencing guidelines, what’s normally referred to as “good behaviour” while in jail/prison can contribute to reduction of sentences. The problem here is what you see quite often though, in that the disposition of the judge plays an important role in determining whether or not it’s applied - specifically in this case, to someone found guilty of stalking and apparently intending to assassinate former POTUS Obama. The former sentencing for participating in the Jan. 6th riots should have compounded the sentence and prevented early release, but this is where the U.S. judiciary is at present unfortunately.