lots of things only have etiquette and no laws.
Laws are nice too, but getting people to follow them is equally tricky.
And regardless, if etiquette is needed for safe and effective sharing of the sidewalks, it's up to these scooter companies to make sure that etiquette training happens before they accept money and turn somebody loose. Otherwise it's classic "privatize the gains and socialize the losses" foolishness.
For example, I almost never see people reserve the passing lane for going fast or passing which is dangerous. People almost never use blinkers. Except, unlike scooters, they are driving vehicles that weigh two tons and are going 65+ mph. And they even require licenses.
So that's a pretty low bar, and not very damning if scooters have some etiquette issues.
If scooters require knowledge, thought, and skill to ride well on a sidewalk, then the two obvious answers are a) require training and a road test, or b) ban them on sidewalks. I don't much care which we go with, but having recently almost gotten flattened by some rental e-scooter idiot on the sidewalk, I think we need to pick one.