Well, I have one of those myself, with statues and not attached to the wall.
However, I'm not that worried, because:
1. I don't have kids etc that may start climbing up it
2. No earth quakes, etc in my area
3. I have some general (non statue stuff) sitting on the bottom which is pretty heavy, just to give it some weight. You could use your heavest statue if you like
4. These come with round screw in feet, one in each corner, the idea is that you can adjust them on any surface to make the unit level, even if the floor isn't.
However, you don't have too, on purpose I made the front feet screwed down more, basically the whole unit 'leans' backwards, towards the wall. So if it is going to fall over all by itself, the first direction its going to fall in is a few cm's till it hits the wall.
So all in all, for me I'm not worried, but I'm sure if you still really wanted to 'attach' it to the wall, then maybe some chain or something to bridge the gap between the wall fixture and the unit.
Here's what mines looks like: