Good question. I'd say based on speed and risk. If they go as fast as a walking person and won't bump people, upset the flow or get in the way, sidewalk is OK (to a limit, you can't have hundreds of these things zooming all over.) If they are slow and on the street or bike lane, they're likely to cause problems. I've heard some people say that companies need to pay to widen the sidewalk to accommodate these things, but that's a whole different problem