I've found you don't generally get far with people by pointing out that their political principles are horrendously inconsistent.

In fact, it's only a tiny minority of people who care whether their political principles are consistent or not.

Given that the purpose of unions is to weaken companies' bargaining position by preventing them from "divide-and-conquering" the workers, I would guess...almost none? And it would be perfectly ideologically consistent?

Who told you a union supporter believes that what is "fair" to workers should also be allowable by corporations?

depends on which union model you take german style codetermination might be ok with that or the laissez faire model not so much