There is no difference. I shouldn't have to join either a union or a company.

> Why must only the owners be allowed to unite for better outcomes?

Well, there's a reason the politico-economic system is called “capitalism” and not “laborism” or “socialism”, and it's not because it is structured for the benefit of labor or society as a whole. Such labor rights as have been established were a small but important step away from pure capitalism, and advocates of capitalism naturally want to peel them back.

A counterargument (that isn't necessarily convincing to me) would be that a union of capitalists is called a cartel, and that is often regulated or opposed, considered illegal or illegitimate; thus a union is equivalent.

I believe there are also libertarians (small-L) who do believe in the above, but think cartels and unions should be unregulated.