1. Public health campaigns - They don't get outraged by junk food, sports cars, etc. because there isn't advertising telling them it's unequivocally bad. Government will tell you that not wearing a helmet, smoking a cigarette, etc. is universally bad so it's easier for people to see someone "doing a bad thing". People get angry when other people "break the rules".
2. They know someone who was affected. I'm guessing if your cousin got hurt because they weren't wearing a helmet, you might carry some emotions that could be released when you see someone else taking the same risk.
3. Human nature. Back in tribes, if our Uncle was trying to scale giant cliff faces for fun, we'd be at greater risk of less food / losing a war if the tribe let him risk his life for little to no tribal gain. Especially if the young kids thought it was cool and wanted to try it too. People are persuadable and can be influenced. It's not unreasonable to think that glorifying death defying sports or financially rewarding them will result in more people risking their lives.
4. Bullying. If you see someone doing something different or not fitting in, some humans feel compelled to yell at that person to increase conformity and cohesion.
5. Low self confidence. They might be highly critical of themselves and see an opportunity to criticize others.
For me, #3 is the most common answer. People in strong, healthy communities feel safer so they probably don't want what they consider a "risky" behavior to spread.