Correct! You must contact your city to deal with it, though if you are just relocating it by a few feet no one is really going to complain. And really, why not let the city accept any liability for any damage if they are willing to?
I don't think it's a stretch to say that the property owner is also responsible for keeping the sidewalk negotiable when somebody illegally dumps a piano on it. From my experience formerly owning property in Philadelphia, it was my impression that keeping the sidewalk clear of anything was my responsibility.
I couldn't find anything about trash on sidewalks in San Diego, but their government page on sidewalks suggest that property owners are partially responsible for maintenance costs of sidewalks:
> Normal sidewalk wear and tear or age damage is the responsibility of the homeowner who can take advantage of the City's 50/50 Cost Sharing Program to help offset the cost of repairs.
https://www.sandiego.gov/street-div/services/roadways/sidewa...
This suggests to me that responsibility for clearing out crap dumped on the sidewalk on your property is probably also the responsibility of the property owner. And if some freak weather caused a blizzard in San Diego, I'd expect property owners would be obligated to clear the snow as well (or pay somebody else to do it.)
I've never had to think about it, but I'm now trying to figure out my options if people left large objects on my sidewalk. I throw away litter. But if there was a bunch of bicycles or scooters, I'd probably ignore them for a bit and hope the owner came back to retrieve them. If it was blocking my driveway or causing a problem, I'd move them off the sidewalk and onto the adjoining grass. If they are left there long enough, I might put a sign to the affect of "this litter will be picked up on __" for the next trash day.
But if I lived in a scooter rental place and this was an ongoing issue, I would definitely be fine with using a service like this.