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, con

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.