Bicycle vs. Sidewalk Bicycle

In reading through the District of Columbia Bicycle Regulations pdf I've noticed a distinction in privileges between "bicycles" and "sidewalk bicycles." For example, a sidewalk bicycle may not travel in the street, unless, for example, it is passing in a crosswalk. So what's the difference between bicycles, which have much more freedom of operation in the District, and these "Sidewalk Bicycles?"

I have found a sidewalk bicycle for sale on Amazon: http://www.amazon.com/Diego-27167-10-Inch-Sidewalk-Bike/dp/B000HDDFFW. It appears to be a very small, childrens' bicycle with training wheels on it. It is quite clear, then, why a bicycle of this size or capability would not be allowed in the street.

But... other than Amazon's result, finding web pages referencing "sidewalk bicycles" proved challenging. In essence, a bicycle is a device with two wheels that propels a user. Maybe a device such as a scooter, which also has two wheels and is used to propel its owner, could be a "sidewalk bicycle" because of its proximity to the sidewalk during operation. In this case too, though, it is quite clear why a scooter would not be permitted operation in a roadway.

The mystery lives on. It is unclear why a term so seemingly ambiguous to even a very educated person would be implicated in a penal code which has the power to affect someone's life. In a document so powerful, Lehman's terms might have been better served.