Their goal is simplicity. If the tutorial looks like I can do it, I am more likely to buy it.
Powering from the arduino is simple and it works in very simple demos.
But as soon as you try to draw more current, it fails.
I suppose that they are showing how it can be done with the fewest components.
And the really cannot cover all combinations of power requirements in a simple demo.
But, Yes, do plan on a separate power supply for the mechanical components.