Actually, this happens only when you're not in airbrush mode, and when the grow over max density box is highlighted. But I think it's a neat little feature, because you don't need to use the scale brush to grow some of your instances bigger, you can do it all with one single brush, in a single mouse click.

Now about painting all sides of a cube: it doesn't work, and I should've known. The eco painter creates a "global ecosystem", which means it isn't linked to nor dependant of any object in the scene, it is only linked to the Vue world. Maybe the manipulation tool can help. Still have to figure this one out.