If people voluntarily attend a rally ... nobody is forcing them to do it.
As such, it makes sense that the organizers would have them sign waivers with "hold harmless" clause.
Otherwise you could wind up with a situation where some jack**s sues the organizer.
"I went to the rally of my own choice, but since I got sick I'm going to sue."
Your examples are why there are laws requiring used car sales to disclose any defects they are aware of ... and car manufacturers to issue recall notices. To inform the public, so they can make informed decisions.
If a car manufacturer issues a recall notice for (say) the air bag system, and you refuse to have it replaced - then you assume the liability, if you are injured by the system.
You are confusing "free will" with "informed consent". They are separate legal concepts. Just because you use "free will", doesn't mean you were fully informed of the dangers/risks. That is the purpose of a waiver. Whether it be for a school field trip, a surgical operation, or a political rally. You are being informed there are risks involved and that you may be taking the liability if something goes wrong.
Now, there's also the fact that it's been upheld in numerous courts that it is virtually impossible to "sign away" some of your legal protections. EG: It doesn't matter if a waiver is signed for a school trip, if the bus driver gets in an accident due to irresponsible behavior.