If they have good arguments against unionization, I don't think it would be necessarily against their interest to have employees hear the other side as well.
That assumes people can tell the difference between good and bad arguments. If you look at the divergence between popular beliefs on economics and the spread of field consensus, that case gets really hard to make. The truth is usually a lot harder to communicate than populist soundbites are.
How are they supposed to even have the chance to tell the difference between the good and bad arguments if they don't get a chance to hear them both?
So people shouldn't be informed about the things they are voting on? Or they should only be allowed to hear pre-approved "good" arguments.
People should inform themselves. People shouldn't be forced to pay to have others give arguments against them.
Well, only one side gets that opportunity at the moment. Would you rather that be the case, that neither side be able to do it, or that both sides be able?