Unions shaping policy is a very controversial subject, and I believe that there is no widespread consensus on whether it should or should not be permitted. It is one of the arguments made against public-sector unions, in that they have an improper influence on government policy, and thus undermine the democratic process.
I thought unions existed to collectively bargain for compensation and working conditions. When working conditions are related to policy, they can negotiate with the employer for 'concessions' (or agreements as the case may be). One (potential) problem occurs when the politicians and the unions agree on something, and enter into contracts with the purpose of 'locking-in' policy decisions beyond the politicians' terms.
I thought unions existed to collectively bargain for compensation and working conditions. When working conditions are related to policy, they can negotiate with the employer for 'concessions' (or agreements as the case may be). One (potential) problem occurs when the politicians and the unions agree on something, and enter into contracts with the purpose of 'locking-in' policy decisions beyond the politicians' terms.