You're missing something here, though. Why are those people deserving of this extra pay? Usually unions represent unskilled workers, but they tend to work things out and strain the system in such a way that they are paid as highly as highly skilled workers. Why should this be the case? It represents a violation of the laws of supply and demand, which is exactly what a monopoly does.
A union is a monopoly on labor. It raises the prices (ie, wages) as high as contractors are willing to pay, which is not how the economy is supposed to work. This drives up prices for EVERYONE to a point where the only winners are the fat cats in the unions who get 30 minute breaks every hour on top of their doubled salaries.
Workers are guaranteed a minimum wage, and those with sufficient experience and training are more sought after, and thus paid higher. That's how the labor market works. Unions break that, such that the entry-level worker is forced to pay dues to subsidize the inflated salaries of other, more long-term workers who may not actually possess more highly-valued skills.
Forced union dues (which are what we're talking about here, not whether unions are good or bad) are essentially a forced tax from a private entity. That should be illegal.