What law specifically are you talking about?
Federal unions have complained that their low union membership totals is due to the federal government being the largest ‘right-to-work’ employer. Right-to-work allows an employee to determine whether he or she wants to join a union. It also keeps employers from making union membership an employment prerequisite.
Via: http://ohmygov.com/blogs/general_news/archive/2009/02/02/a-l...
See also:
However, the most important aspect to having a federal union membership is the collective bargaining agreement. This agreement is a contract between an employer and the union which terms of employment are negotiated and adhered to by the employer and the union. These terms can be about the hiring process, salary, working conditions employment termination, work hours or employee grievances. A union represents the employee to ensure that the employer does not violate the collective bargaining agreement.
There is a long list of things up for negotiation. It seems to me that very little is being forced on anyone.
and
Also, federal unions cannot compel a non-union member to become a union member; to pay union fees or make union membership an employment precondition. As a result, federal union membership numbers are relatively small in comparison to the overall size of the federal civilian workforce.
Again, no one is being forced into anything again. They can't even establish union only jobs.
via the same link.