How can they let him get away like that?! No freaking way
Lewis has a mental-type Soot Power that, as shown with their interference on Louise/Lou's Unification Ritual, has the power to essentially hypnotise humans to their belief. Faceless Dolls too are humans, so Lewis can use his power to disarm and nullify the threat of those arresting him, make them simply stand still staring at a wall.
In general it is easy for other Adult Shadows to overpower Faceless Dolls via their abilities, but normally such resistance is
pointless because they'd have nowhere to go even if they escape, no allies in the Children or Grandfather Wings or outside the House for one branded a criminal. It's why Joseph thought the matter was settled.
But if Lewis
does have sanctuary with Anthony in the Children's Wing, that's a different story, in which resistance is worth a go. It's why when Joseph made the connection after Kate's account, he realised he had to go back and check.