I hear commercial vehicles using jake brakes, aren't they unlawful?

"Jake brakes," defined as a hydraulic engine attachment which converts a diesel engine into an air compressor and when engaged operates to slow the vehicle, are permitted while traveling downhill on the following roadways in Homer;

The Sterling Highway from Milepost 169 (top of Baycrest Hill) to mile 173 (intersection with Pioneer Avenue), Main Street, East Hill Road and West Hill Road.

HCC 7.06.020

A first violation is a $100 fine. A second violation within 6 months of the first conviction is a $200 fine plus proof of satisfactory Homer Police commercial vehicle inspection.  A third violation within 6 months of any prior conviction is a $300 fine plus proof of satisfactory Homer Police commercial vehicle inspection.