In general (and the situation in transportation industries regulated by the Railway Labor Act may well be different), in private industry, under Supreme Court decisions made under the National Labor Relations Act and the Taft-Hartley Act, workers engaged in economic strikes can be permanently replaced (although prior to about 10 years ago this was hardly ever done). Workers engaged in Unfair Labor Practice strikes (subject to determination by the National Labor Relations Board) cannot be permanently replaced. I disagree, of course, that the ability to replace strikers levels the playing field. It does very much the opposite. The field would be level if you could strike without danger of being replaced permanently.