Yes bicycling is legal on most Michigan "sidewalks". Ours might be better tagged as highway=path + surface=concrete + width=1. Most Michigan mappers only add bicycle=yes where it is needed to route someone where the road network will not. Like, a cut through between two neighborhoods. Another reason I do not do add bicycle=yet to sidewalks, besides limiting routing confusion when it would appear to the router as bicycling paths on either side of the road, plus the road itself, then some roads will also have bicycle lanes on that road; I hear that high speed cycling on sidewalks is statistically more dangerous.