Still, I just can't support corporate greed in any form with a good conscience
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"Corporate greed" is little more than an expression of self-interest (or, if you prefer, selfishness). A lot of people think of it as distasteful - thanks, in large part, to state propaganda - but it leads to positive outcomes in a society based on voluntary exchange. It can also lead to negative outcomes as the result of state coercion.
When folks make voluntary arrangements with each other, they are driven by their self-interest. Both parties want to make their lives better in some way. If either party is made worse, we can assume the exchange would not occur.*
Here's an example. Let's say I go into Starbucks to get of cup of coffee. A "venti" costs $2.15. I am willing to exchange that amount of money for the coffee. Starbucks is willing to exchange to coffee (plus labor, materials, insurance, utility payments, etc.) for the cash.
My life is made better by the exchange. It's safe to say the exchange works out toward a better end for Starbucks, too (for their employees, shareholders, contractors, etc.). Both parties are driven by self-interest, and both parties come out the other end better for it, according to their own evaluation. (Yes, even if their battery acid ends up killing me.)
What is true on a small scale should be true on a larger scale. And indeed, we can see that in our daily lives. We enjoy products from Apple, Microsoft, Toyota, and Google due to "corporate greed," or self-interest. We enjoy haircuts, manicures, and massages due to the vendors' self-interest and the "greed" of the companies for which they work. We enjoy this forum because a company made the server, and another company sold the space, etc.
Now, on to labor. Labor is essentially a product sold by the employee to the employer. In a society based on voluntary exchange, we can assume the employee has no better options (based on his personal assessment of self-interest) than the one he now enjoys. That assessment might include salary, vacation time, location, and other perks.
If the employer, the other party in the exchange, is coerced into paying his staff more, giving more vacation time, or extending numerous perks and benefits, he will consider other options. He must to pursue his self-interest. That might include cutting hours, eliminating jobs, etc. If such options exist, he is likely to take them.
Unions, by themselves, are fine as long as they do not use aggression. They are a platform where employees and employers can negotiate based on their respective self-interest. Of course, the history of unions is riddled with laws, violence, and other acts of aggressive coercion. That's bad for everyone, even if the results are difficult to see on the surface.
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* Yes, there are exceptions, such as one party's kid being held for ransom, but that takes us of way off the path. Let's ignore such exceptions.