The main objection that labor is likely to raise to this is that management is the enemy because management does not have the best interests of the labor union or its members at heart.
In many cases, labor unions feel that the employers do not want what is best for workers. They feel the employers will always seek to benefit the company even if that hurts workers. Because they feel this way, unions might feel that they are likely to hurt themselves if they engage in any cooperation with management. They will feel that the management's desire to cooperate is a trick to con the union into agreeing to something that will hurt it.
The previous post is quite accurate. One of the most pressing concerns for all labor organizations is how to walk the tightrope between cooperating with management in order to "get things done" and appearing to its membership that it is a staunch advocate for their needs. It is a balancing act because if union leadership appears too close to management, charges of collusion and a lack of faith amongst the members might result. At the same time, if union leadership is too antagonistic with management, the result could be nothing getting done and the membership suffering. It is a very precarious balance. Union leadership has to have very good credibility and standing with its workers to ensure that there is a faith present so that any arrangements struck with management is not perceived as "selling out the union" and rather a full understanding of how business needs to operate to make sure all sides benefit.