Were he in any other city, we would not have let him leave.
However, you have to respect the Shi'a culture in southern Iraq. They've helped the US more than most know. Sure, they're infiltrated with Iraninan agents, but the situation in the SOuth could be 100X worse.
That said, this guy (al Sadr) is on the ropes. He lost a major endorsement when al Sistani pushed him from Najaf.
I only hope our forces have strong actionable intel in his new city. The US troops have enjoyed substantial actionable intel from the locals (could be more, but certainly could be far less if the locals truly wanted Sadr).
Finish the job so our troops can focus on Baghdad and the Sunni Triangle.