It is the policy of the commission to promote bilateral collective bargaining negotiations between employers and the exclusive representatives of their employees. Parties are encouraged to engage in free and open exchange of proposals and positions on all matters coming into the dispute between them. The commission deems the determination as to whether a particular subject is mandatory or nonmandatory to be a question of law and fact to be determined by the commission, and which is not subject to waiver by the parties by their action or inaction. It is the policy of the commission that a party which engages in collective bargaining with respect to a particular issue does not and cannot confer the status of a mandatory subject on a nonmandatory subject.