What's up w/all the new/student driver stickers on cars?

What is up with the increasing number of new/student driver stickers on cars? (These are the ones that also say be patient, caution, etc.)

I see dozens of them every time I'm in Issaquah. Is DOL requiring these now or something?

Help me understand. Or is this one of those un-understandable mysteries like La-Z-Boy?