https://jsfiddle.net/8vhn8fry/
I'm not sure if this is a bug or I'm just using it wrong.
Try click on a category, and then right click on the map to go back to All Categories. Now click the redraw button. Instead of staying in "All Categories" view, the redrawn treemap will be at the sub-category that you just clicked on.
On a sidenote, treemap API has a lot of additional methods: https://developers.google.com/chart/interactive/docs/gallery/treemap#methods
How do I access these methods in google-chart Polymer wrapper?