When a group of businesses decide to fix their prices, that is not compete against each other, they violate many nation's laws and can end up in jail. When a group of workers do that by authorizing a labor union to bargain "collectively" with their employer over pay, they are fixing the price for their labor.
Thus collective bargaining is a form of price fixing and should be banned.
The labor group represents the employees who provide the demand of labor that the employer needs. The employer demands that labor so they offer a price. Feeling that they are more productive the labor group demands a higher wage or better benefits they feel are more suitable. Negotiation is not a form of price fixing.