If you notice what is going on in earlier passages, the salting and making barren of the land is part of a wholesale judgment already spoken about.
That area was laid waste and not rebuilt until much later when the land had recovered.
When I get confused about why something is so, I like to check the surrounding passages for clues ... nothing is really random in the Bible. It can just be confusing to sort out the significance amidst competing details.
Another dirty deal for security and profit and trusting in the people and the land that offered it wasn't part of what should have been done. Treachery begets treachery.
This judgment shows you really can't trust in the ungodly people or the land, only God.
7 When it was told to Jotham, he went and stood on top of y Mount Gerizim and cried aloud and said to them, “Listen to me, you leaders of Shechem, that God may listen to you. 8 z The trees once went out to anoint a king over them, and they said to the olive tree, a ‘Reign over us.’ 9 But the olive tree said to them, ‘Shall I leave my abundance, by which gods and men are honored, and go hold sway over the trees?’ 10 And the trees said to the fig tree, ‘You come and reign over us.’ 11 But the fig tree said to them, ‘Shall I leave my sweetness and my good fruit and go hold sway over the trees?’ 12 And the trees said to the vine, ‘You come and reign over us.’ 13 But the vine said to them, ‘Shall I leave my wine that cheers God and men and go hold sway over the trees?’ 14 Then all the trees said to the bramble, ‘You come and reign over us.’ 15 And the bramble said to the trees, ‘If in good faith you are anointing me king over you, then come and take refuge in my shade, but if not, d let fire come out of the bramble and devour e the cedars of Lebanon.’
16 “Now therefore, if you acted in good faith and integrity when you made Abimelech king, and if you have dealt well with f Jerubbaal and his house and have done to him g as his deeds deserved— 17 for my father fought for you and risked his life and delivered you from the hand of Midian, 18 and you have risen up against my father's house this day and have killed his sons, seventy men on one stone, and have made i Abimelech, the son of his female servant, king over the leaders of Shechem, j because he is your relative— 19 if you then have acted in good faith and integrity with Jerubbaal and with his house this day, then k rejoice in Abimelech, and let him also rejoice in you. 20 But if not, l let fire come out from Abimelech and devour the leaders of Shechem and Beth-millo; and let fire come out from the leaders of Shechem and from Beth-millo and devour Abimelech.” 21 And Jotham ran away and fled and went to Beer and lived there, because of Abimelech his brother.
The Downfall of Abimelech
22 Abimelech ruled over Israel three years. 23 And God sent an evil spirit between Abimelech and the leaders of Shechem, and the leaders of Shechem o dealt treacherously with Abimelech, 24 p that the violence done to the seventy sons of Jerubbaal might come, and their blood be laid on Abimelech their brother, who killed them, and on the men of Shechem, who strengthened his hands to kill his brothers. 25 And the leaders of Shechem put men in ambush against him on the mountaintops, and they robbed all who passed by them along that way. And it was told to Abimelech.