If you need to qualify to join a union, or to do a specific job in a union, the qualifications will be owned by the companies which are politically powerful.
Imagine being unable to qualify as a C++ programmer unless and until you've qualified on Microsoft Windows and Visual C++.
Imagine being unable to qualify as a Perl programmer at all, because Perl isn't one of the technologies owned by a major corporation.
And, of course, doing freelance work, or contributing to Open Source, makes you a scab, stealing work from Poor, Honest Union Workers.