The Driver's license requirement for MBs in many states was added when it was noticed a lot of people who had lost their licenses, especially because of DUIs, were simply switching from cars, trucks, motorcycles, etc to driving motored bikes. Losing their license was both a punishment and a way to prevent them from continuing to drive under the influence and endangering themselves and others, so circumventing the rules and continuing to drive by switching to a MB is prevented by requiring a license. I hope this helps you to understand the rational behind requiring a driver's license to operate a MB.
Is that true? I'm asking because I assumed they just wanted to prevent kids from injuring themselves.
If the objective was to punish those who lost their licenses, that's dumb. On a MB, they're not much danger to anyone else. It's enough punishment that they can't drive. As a society, we are better off if they are still able to get to work/school/wherever.