The legal situation in Israel
I mention Israel first since am involved in a legal case before the Israeli national labor court on this very question: Arguing that those PhD candidates who are required to work all week long, who are forbidden from working outside the university, who can be required to teach up to a limit according to the needs of their academic departments etc. etc. - who contribute significantly to the research (and teaching) "outputs" of the university, should be considered its employees, and the monthly "stipends" that they get should be recognized again as their salary. This is case ע'ע 5439-04-16 (Labor Appeal 5439-04-16). I've authored a booklet on the history of this struggle, which has an English version:
Or get the Hebrew version; in fact, if you can read Hebrew, you can find the court case documents here.
The legal situation in the US
Another, perhaps more relevant, legal treatment of this matter can be found in the recent ruling of the National Labor Relations board, in the case of Graduate Workers of Columbia University, UAW, AFL-CIO vs the University of Columbia (NLRB 364-90: HTML PDF, all case files): The court has found that, indeed, Graduate students who perform meaningful research or teaching work as part of the course of their PhD program (and are paid), are considered university employees to the extent of the applicability of US labor law, especially the laws governing unionization and collective bargaining.
That decision reversed the unfortunate (3-to-2) decision in the 2004 case of Brown University (NLRB 342-42: HTML, PDF), which had itself reversed the 2000 decision in the case of NYU (NLRB 332-111: HTML, PDF).
See also the AAUP description of the Columbia U case and their position.
The actual answer
I can talk for hours, literally, about this subject, and I actually have... but in a nutshell:
- Being an employee and being undergoing a process of education are not mutually exclusive; and you always learn when you're starting out in a new career path, be it in industry or in academia.
- The common labor-legal criteria for being considered an employee can and do apply to many/most/all PhD candidates (depending on which state in the world, which kind of PhD work etc.)
- A university is an organization whose ongoing objectives are to produce research findings and to teach students. If you contribute towards those goals (by performing research and teaching), even if you're also undergoing a process of education yourself, you should be recognized and remunerated as such.
- Post-Docs only undergo any sort of education through their practical research work (which, of course, involves learning - but that's the case also for the most high-ranking Professors); they don't take classes, get homework and take exams to see whether they've studied well enough. so about them there should not be any doubt of being in relations of employment.
as for why have PhD candidates when you can have Post-docs - I've already written enough, let others discuss that :-)