https://web.archive.org/web/20180623224226/http://www.fdot.g...
[0] https://www.forbes.com/sites/carltonreid/2019/05/10/cyclists...
[1] https://usa.streetsblog.org/2018/01/03/study-cyclists-dont-b...
In the US, for experienced/aware/agile cyclists who ride often and defensively, it's usually safer for them to not wear a helmet because:
i) Vehicles statistically give helmet-less cyclists more space.
ii) Unconscious personal safety anxiety and risk-aversion that comes along with a rational cyclist not wearing a helmet. Something they must never forget.
While there are imperfect operators of every mode of transportation, don't forget that also applies to drivers and pedestrians.
Aside from that, riding on sidewalks is legal in Austin. You clearly aren't familiar with the situation.
Also, they take up less room. The density of cars is ridiculous. You have narrow sidewalks with hundreds of people and then 5x the space for 10 people in one car each. Not to mention the cars with no people at all just sitting there taking up space.
It's legal to ride bikes on sidewalks here.