The union really should have no interest in opposing a worldwide draft, considering that the players subject to the draft would not be members of the union. Current union members would be better served by not having the unrestricted competition from foreign players. You could make an argument that all players benefit from higher pay going to non-union foreign players in that higher pay affects arbitration awards, but that is a trade off for dilluting the players' bargaining power up front. It is a stronger market for players' market if you have more teams chasing fewer players.
The reason the union wants concessions for an international draft is that it would hurt players' agents.