tree.dndController.singular = true;
これが正しいかは、ちょっと怪しいが、自分でも試したら動いた。
dojo - How to disable multiple selection of nodes in dijit.Tree? - Stack Overflow
var tree = new dijit.Tree({
id: "tree1",
model: model
}).placeAt(win.body());
tree.dndController.singular = true;
