[wolf78]

Since when should companies be forced to sell products from all providers?

The problem is when a monopoly hinders others from entering a market. One example: Let's assume I run a fictional autoworkers union and have valid contracts with both some big Midwestern automakers and suppliers around the county. Now let's assume a Japanese competitor, Toyohonda want's to build a factory in non-unionized Texas, but needs suppliers. But due to my contracts I can prevent these suppliers from cooperating with Toyohonda, either forcing the Japanese out or forcing them to get unionized.

[M203M4]

In this example, why should Toyohonda be in any way able to use the force of government to interfere with the contractual terms between other private entities? Interference at defacto gunpoint in such matters is an assault against freedom of association (as are unions whose creation is based on votes instead of a sum of voluntary associations, but that is another matter).