I get the property rights angle, but I'm not sure what you mean by liability.
Liability insofar as the Oklahoma legislature is removing Weyerhauser's right to set whatever policy they deem the most appropriate in order to protect their employees from work place gun related violence.
The legislature has effectively voided their ability to make a choice, but if a work place related shooting takes place, Weyerhauser is still financially, legally, and perhaps even criminally accountable to the victims of the shooting for a shooting that could arguably have been avoided.
In other words, if I'm not allowed BY LAW to do whatever I believe I need to do in order to stop you from jumping off the roof of my house and into my pool, why should I be liable for your injuries if you do it and get hurt?