I've owned both a scooter and skateboard for 7 months.
I disagree, you can ride a scooter on a sidewalk safely and I need to do it from time to time. Of course, I don't prefer it. But it's pretty easy to do safely and it shouldn't bother anyone if done correctly.
The rules I follow:
1. Go much much slower.
2. Stay far away from other people and blind corners.
3. Always assume a person is about to make an erratic movement
4. Never pass closely to a person. When passing, go their walking speed + 1mph. It should be a similar speed differential to passing someone when you're walking.
5. If you have no space to pass, you need to go walking speed
until space opens up.
6. if the sidewalk is too packed, just get off and walk it.
7. If you have far visibility (no blind corners, doorways, storefronts, etc) and there is no one in sight, let'er rip.
It all depends on the conditions. You should never blow by people at 15 mph. I commuted partly on 5th street in downtown Los Angeles and never had an issue, a close call, or had someone bat an eye. The goal is to blend in with other people walking.