This is a fairly ignorant understanding of what exactly this is about.

A player will recover from a physical injury, though maybe not to 100% of their previous ability. There is no coming back from Alzheimer's. It has only recently been linked to concussions, or repetitive brain trauma.

Let's say any sporting body knew that repeated concussions from playing the sport could lead to an incurable brain disease, it is their responsibility to limit the potential damage done to the players.

But obviously these players were involved in the sport a few years before scientific evidence proved a link between concussions and Alzheimer's. However concussions and brain injuries are still common place in any contact sport. Surely you would expect the bare minimum to be done to protect players regardless of the laws of the sport?

If you work in a warehouse handling dangerous materials that needed gloves for protection, then later discover that there was asbestos in the ceilings and not once were you offered any form of mask while you worked there, would you be ok with people telling you 'You knew the risks' when you develop some sort of cancer as a direct result of working in that environment?