I'm not sure the point is clear here. Set aside the "objective danger" concept for a second. Even if everyone in the most popular area of the park wore helmets, do you think they would want to stick around in a rain of pea gravel? And if you do consider the rockfall hazard, do you think a helmet would even help if the odd baseball-sized piece came down from up there? Is it so unreasonable to at least show some consideration, and maybe hold off until the park is less crowded? That's all.
The two who did this climb seem like really cool people, genuinely good people with no intentions of harm. The climbs skyclimb has reported on have been inspiring and cool, as is this one. It's great to see the climbs being done, and the fun that has been had in the spirit of adventure. Unfortunately, they now see some people were a little ticked by the timing and location of this last one, and perhaps in the future they will not do it on a busy Spring weekend, or at least consider that fact before setting off.
But after all this, to just say f'em all I'm gonna have my fun this sunny weekend, even if there's a convention down there, well that seems inconsiderate. To suggest that a helmet should be worn at all times within say 100 feet of the walls strikes me as a bit disingenuous, even if ultimately, it would be the safest option. The fact is, there will be people down there w/o helmets on, and even if they wore them, no one likes rocks thrown at them, of any size. But maybe that's just me.