[davmre]

In a healthy, well-regulated market, corporations compete to provide products and services. This provides everyone with incentives to improve quality and lower costs, and prevents a single corporation from charging monopoly rents, which is good for society.

Unions do not compete with each other. In fact, the entire point of unions is to eliminate competition in the labor market. Unions have a government-sanctioned monopoly on certain types of labor, and the existence of a monopoly drives up prices and reduces the incentives to perform quality work. All else equal, this is bad for society.

Of course all else is not equal, there are lots of positive aspects to collective bargaining - it does help rectify the otherwise-significant power differential between individual employees and large corporations - and there are reasonable arguments that the labor movement and unionization in general have been massive drivers of improved human well-being over the last century. But that doesn't mean that unions and corporations are two sides of the same coin -- they play very different roles in the market and have very different regulatory considerations.