Why liberals think libertarians hate unions: they raise wages and improve working conditions for their workers at the expense of profits.
Why most libertarians I know who hate unions actually hate them:
Why isn't everybody doing this, since the technology works and has been proven overseas? There are legitimate concerns to be weighed. Unless the toll road already has high transponder market share, some fraction of cash customers may simply stop using the toll road if the cash option is eliminated. There are also real costs (staffing and technology) involved in video license-plate recognition and billing. And there is the problem of what to do with all the now-redundant toll collectors, especially if they are unionized.
We should not be prevented from implementing productivity enhancing improvements that make everyone better off, because a handful of people would rather be paid inflated wages to do it by hand. Yet more and more often, this is at the heart of union disputes--the infamous dockworkers strike a few years ago was centrally a dispute not about wages, but the fact that the ports no longer needed to employ so many dockworkers. Or look at all the workers that the Big Three auto workers are forced to pay to sit there on the off chance that they might someday be needed. Obviously this is not the only thing unions fight about--witness the current writer's strike. But to me, the central problem with unions, to the extent that there is a problem, is not that they demand higher wages, but that they reflexively oppose productivity enhancing change.