They are grown adults. Not up to us to question their decisions. When we step into the box we can decide for ourselves.
I take no issue with college players not wearing masks. I wish more of them would consider it, but I understand not doing it at that level. And yes with the rare exception of a 17 year old freshman they are all legal adults who can make their own decisions.
With respect to this particular play I'm pretty sure a mask would have prevented nearly all of the damage that was inflicted. Though I can't say that with 100% confidence.
And this seems like a really freak play that you could easily go 10 years and never see again.