“Do not pray for this people nor offer any plea or petition for them, because I will not listen when they call to me in the time of their distress. Then the LORD said to me, “Do not pray for the well-being of this people.

Then the LORD said to me: “Even if Moses and Samuel were to stand before me, my heart would not go out to this people. Send them away from my presence! Let them go!

So apparently Jeremiah keeps on praying for mercy. But God knows the people of Israel won't listen to him if he doesn't actually punish them. It was something that needed to be done. I think it's sweet that Jeremiah pray for them like that.