Not to put too fine a point on it but because sidewalks are for walking not parking. If they were meant for parking , then they'd be called sideparks. We've been given the privilege of parking in places, where cars have to pay. Flouting the law just makes us look bad, making it possible that this cash strapped city might withdraw that permission.
The pictures, on this site, don't look like people are parking on the sidewalk.
http://ukfrancebikers.com/2010/11/16/how-to-park-your-bike-in-westminster/