Any legislation which gives an advantage to unions by coercing employers to act against their own interests has lost the moral high-ground in my opinion. I will support unions as long as any negotiations are voluntary, and any pressure a union can apply within rational laws which protect against coercion is fine by me, as long as owners of business or those given authority can fire people at will. I think that labor fighting for its own interests is part of a free society -- but coercion isn't. It might sound cruel in today's world to say that employers should have a right to fire anyone they wish to fire, but there are other more effective ways for unions to make their case and protect their interests than lobbying government to force employers to act against their, the employers', own interests. It wouldn't be moral to force unions to not strike, and it's not moral to force employers to negotiate, and to prevent employers from firing those they wish to fire.
Employers wouldn't fire willy nilly if they have the right -- they would sit down with honest brokers if it's in their interests, and unions would have to show where both sides benefit -- otherwise, it's simply a system of unions bullying employers to get what they want regardless of what is good for the employer. Without the ability to refuse to negotiate or to fire those who appear to the employer dangerous to the employers' going concerns, it's a one-sided route to labor benefitting at the expense of employers, and this leads to bad financial situations as we've witnessed.