Such rules come in at the state level and are usually pushed for by the large national unions to bully and push out local and regional small unions. Every major union from UAW to SEIU has been guilty of this.
This is not true. Unionization based on enterprise-level votes was put in place with NRLA in 1935. The NRLA and Taft-Hartley would have to be amended or replaced to change this. I have no idea if large unions have ever lobbied against replacing them at the Federal level, but there hasn’t (in recent history) been any serious push to do so that these unions would oppose.
