Steroids in Baseball

One thing I can't understand about the steroids in baseball controversy: Why is the players union the one opposed to testing of the players? It should be in the interests of players to support testing and for owners to oppose it. After all, owners benefit from the increased excitement of the game generated by more home runs. Players suffer because the prevalence of steroids forces players to risk serious long term damage to their health just to remain competitive. The players union is so blinded by opposing everything the owners want, that in this case they do not even realize that the owners are acting in the best interest of the players.