If you're interested in maximum freedom of choice, didn't people clearly choose that unions are supported, in large enough numbers to get it written into lawbooks? Or are businesses somehow more free than people, so laws don't apply to them?
I think you're confusing the freedom of the majority to repress the rights of the minority, with "contract freedom", i.e. the right to free assocation.
And yes, I am in favor of limiting that. It has been used for many, many abuses through history. "Free association" really only exists when both sides have equal power, and oh boy is that not the case in most cases.