If you were trying to hire Jimi Hendrix to play guitar for you, would you make him audition by playing Mary Had a Little Lamb? Don't dictate what they should play, let them show you what they got.
This is sort of like how if you want to hire a chef for a Michelin star restaurant, you ask them to make an omelette, not come up with an innovative dish.
It's something that virtually everyone knows, and is a test that they have a very solid command of the basics as a foundation to build more complex things on.
