"Everyone has the right to form and to join trade unions for the protection of his interests." - United Nations Universal Declaration of Human Rights, Article 23.4.
Scott Walker signs the Right To Work law in Wisconsin.
Nobody's saying you can't form a trade union. They're just saying you can't force people to join it. If you can deliver on your promises to improve pay and working conditions in a serious way, you can probably get people to sign on. Unions did this for years. Yes, it's harder to do it this way than with a rule that requires workers to join your union; yes, it's harder to do it this way than with rules forbidding the company from hiring scabs. But you can still do it, if you devote yourself to providing returns for the workers who pay dues. You can still make your case to the people, too, who may refuse to cross a picket line if they agree you are being shabbily treated.
It was too easy for a long time. Many unions did badly by their members, and devoted themselves to power and corruption instead of helping their own loyally as the first priority. If you want to survive, you'll have to get back to what made unions strong in the first place.
Wikipedia says that "scabs" in this context dates to the Elizabethan era, by the way. I had no idea.