Are there any laws that stop management from forbidding their workers to join/form an union? Because it seems that all it takes is them refusing to hire someone who’s part of an union or putting on the contract that they can’t join and unions are dead.

Technically, the Wagner Act forbids both of those practices. The problem is that there’s not enough teeth when it comes to the enforcement of the Wagner Act, so that companies often simply budget in the fines for firing someone for being a union activist as the cost of doing business.