In most cases the NLRB says that a change has to have significant impact on employees in order to trigger the bargaining rule. For many years the policy of the union was to say no, the union doesn't recognize company work rules or absentee policies. What we recognize is that any discipline must follow the discipline clause of the contract. The situation is more complicated now because of the NLRB and courts whittling away union rights. Now the NLRB says the boss should bargain with the union over work rules and absentee policies.
Many arbitrators now consider employer written work rules and policies to be valid and enforceable.
Every situation is different, but in most cases it is best for the union to mobilize the members and fight for the best rules or policies possible. This does not mean that the union is condoning the rules or policies. The union is doing its job in trying to protect the members from unjust treatment. Treat this just like contract negotiations and take time to mobilize the members for a fight around the issues. Try upping the ante on the employer.