In theory it's exceptionally illegal to curtail unionization efforts.

In practice, the law has been whittled away by decades of conservative judiciary decisions and weak department of labor enforcement. This isn't helped at all by the balance of power.

Companies can afford to scare off some degree of workers, especially at the lower end of the salary range. Big businesses can survive shutting down a store or losing business at locations indefinitely. Big businesses can afford expensive lawyers and to indefinitely stay in litigation over union busting efforts.

For workers, it's a completely different proposition. Is Walmart or Home Depot or Starbucks going to want to hire someone that is actively suing another major corporation for anything at all? It's even worse if it's labor rights related, but just suing them in the first place is going to make it a struggle to find employment at a lot of places. That's even pretending they can find & afford lawyers. Or that they can handle the transition period from job A to job B even if it isn't difficult to find job B.

These businesses hold all the cards and they know it. You see similar thinking, though different details, behind Hollywood's decision to just try and wait out the striking writers and actors. They can survive losing billions of dollars in income a year from now with unmade projects; striking workers will struggle to get by with no salary.

There are, but since Joy Silk Doctrine was abandoned in the 1970s, the NLRB has been severely handicapped in defending and certifying unions. It's made worse when generally the penalty to the company is backpay and offering the employee their job back. The PRO act has been introduced to Congress several times and includes the ability to leverage fines of $50k per person affected per ULP, but has been rejected by the Republican party which has a firm anti-labor stance

IANAL - anecdotally businesses here ask forgiveness rather than permission, and penalties are a pittance. They usually do not even have to admit guilt in court. An individual may win their case, but their advasary has its own dedicated legal department and comparatively infinite resources.

The legal barrier to prove union busting basically means little recourse will be made.

If they get sued for it, they'll probably settle for less money than they figure they'll lose if employees unionize.

I'm currently sueing a major corporation for violating labor laws and they asked for mediation (which means they want to settle and keep it out of court). I didn't even hire a lawyer. Just lodged a complaint with my state department of labor and it went from there.