“Because with lies you have grieved the heart of the righteous whom I have not made sad, and strengthened the hands of the wicked, that he should not return from his wicked way and be saved alive. Therefore you will no more see vanity, nor divine divinations, and I will deliver my people out of your hand. And you will know that I am Yahweh.”
The prophetesses had done the very opposite of the purpose intended for them. Instead of making the righteous glad and the wicked aware of their sins, they had by their lies grieved the hearts of those Whom Yahweh wished to make happy, and had strengthened the wicked in their sins, removing from them the fear of temporal judgment. Thus the wicked continued happily on in their wicked ways, instead of repenting and finding life, and the righteous grieved when they should not have needed to.
This was why Yahweh would bring His judgment of death on the prophetesses, so that they could no longer see what was vain and empty and divine what was false, to the hurt in the end of both righteous and wicked. So were they, and we, discovering why God's judgment on Jerusalem was going to fulfil His purposes and was in line with His justice and holiness. Then all would know that He really is Yahweh, the holy and living God Who acts.
There is little difference between these prophetesses and those who in our day go to the occult to receive advice, discover the future and contact the dead, and indeed even in some cases to bring harm on others. These practises are equally condemned.
So God was building up a picture as to why Jerusalem had to be destroyed. He had outlined the detail of the different forms of idol worship going on in Jerusalem (Ezekiel 8:5-18) and its surrounds (Ezekiel 6:1-7), which involved both priesthood (Ezekiel 8:16) and laity, with its resulting descent into all kinds of wickedness, He had described the evil ways of the civil leaders, with the resulting violence (Ezekiel 11:1-13), and now He had demonstrated the evil of the false prophets, and the wicked practises of the prophetesses. The whole city was a mass of wickedness, ripe for judgment.