It is taboo to ride a bike on a sidewalk—especially when there are children and elderly walking on it—but when it comes to "multi-use" paths, such as the Lakeshore path, it is officially okay. A multi-use path is at heart just a sidewalk on steroids.
Sharing on the sidewalk is verboten:
But sharing a narrow multi-use path or bridge is perfectly fine—and officially promoted:
Bottleneck on new Portland Street Bridge.
In fact, (many) planners and architects happily propose and design new multi-use paths that force meandering walkers to interact uncomfortably close with commuting and recreational cyclists. The result is a trail that serves neither group well: parents have to constantly keep their children in check lest they make a beeline across a cyclists path; and cyclists have to slow down to a walking pace, or swerve around meandering pedestrians walking side-by-side. Hardly the best use of this most efficient machine (No joke: "In fact cycling is more efficient than any other method of travel--including walking!"). Yet our planners, for most of their off-road projects, continue to just squeeze bikes into this shared space.
A multi-use path is not a bike path. It is a glorified sidewalk.