The “free assembly” is very debatable as most current unions are organized to blackmail the businesses that they work for. Many states allow the formation of unions but some state do not allow them so I am not sure if the “free assembly” argument in this case can hold any power.
I think that you are referring to laws involving strikes and collective bargaining. Many states have outlawed strikes by public employees although those bans are not enforced much. Some states do not allow collective bargaining. I support laws against strikes and collective bargaining for public employees because the taxpayer is at a huge disadvantage in these situations.
If a company wants to bargain collectively, the company should not be forbidden to bargain unless the bargaining creates a monopolistic situation in the industry. I am confident that few companies would engage in collective bargaining if laws did not compel them to bargain. Unions are monopolies that are created and maintained by legislation.