There's a huge difference between "argue against" and "actively try to sabotage".
Here (Italy) unionization is a constitutionally protected right. My employer can try to persuade me not to join one, but is absolutely not allowed to retaliate against me if I do, discriminate basing on union membership, etc.
This isn't a true statement.
Companies are allowed to argue against unionisation. In the US isn't it even a constitutionally protected right to do so?