Well here is what I found for the Chicago city ordinance on riding bikes on sidewalks:

9-52-020. Riding bicycles on sidewalks and certain roadways.
(a) No person shall ride a bicycle upon a sidewalk within a business district.
(b) No person 12 or more years of age shall ride a bicycle upon any sidewalk in any district, unless such sidewalk has been officially designated and marked as a bicycle route.

actually, here is the latest amendment:

9-52-020 Riding bicycles on sidewalks and certain roadways.
(a) No person shall ride a bicycle upon a sidewalk within a business district^
unless such sidewalk has been officially designated and marked as a bicycle route, or
such sidewalk is used to enter the nearest roadway, intersection or designated bicycle
path, or to access a bicycle share station.
(b) No person 12 or more years of age shall ride a bicycle upon any sidewalk
in any district, unless such sidewalk has been officially designated and marked as a
bicycle route, or such sidewalk is used to enter the nearest roadway, intersection or
designated bicycle path, or to access a bicycle share station.
(c) Bicycles shall not be operated on Lake Shore Drive or on any roadway
where the operation of bicycles has been prohibited and signs have been erected
indicating such prohibition.

So the rider was disobeying the law.