Each side is free to do as it pleases as long as they don't defraud, steal from, or assault each other. Today, most employers are legally prevented from hiring new workers during the strike. Union violence is often ignored. Thus, unions have the edge in any negotiation and union wages are usually artificially high. Businesses on the edge go under and the disadvantaged workers lose their jobs to subsidize the union members.
Monopolies are very rare in the free market, contrary to popular dogma.
For all his greed, Rockefeller could only dominate the marketplace by giving the customer a good deal. Government intervention is usually necessary to make monopolies possible. We have too many rapes in the United States as well. Just as a potent man can be a rapist or a lover, so too can guns save lives or take them. Fifty percent of murdered spouses are wife beaters killed in self defense.