That is one way to look at it. However, the business has a greater duty to provide a safe environment and take steps to avoid foreseeable dangers created by their policies. Every mass shooting has taken place in a place that bans guns. Several perps are on record as explicitly picking a place guns were banned to do their crime. By banning guns they vastly increase the danger to patrons and employees.
The customer or employee does not have the same responsibility as the business to know the risks of entering the establishment. For example, amusement parks budget for a certain number of injury wrongful death payouts every season. The patrons don’t have as great a responsibility to know the extent of the possibility of injury or death as the park,
My post is based entirely on the legal principle related to mitigating risk to a plaintiff, not on the practical application of defendants making payments under civil lawsuits. What I have described here is called the "doctrine of avoidable consequences" in tort law. Yes, the owner of the establishment might be liable for damages. But if the plaintiff in a case related to a shooting like this already knows what he's getting into, any damages might be reduced -- or even eliminated entirely -- in a civil trial.