Primarily to insure they tax them all properly. :-)
But there's something to be said for labor law as a moderating force between employers and employees as well -- things get ugly fast when there are no legal requirements insofar as how workers are treated.
It doesn't (necessarily) -- this is all tied up in the labor law reforms of the late 19th/early 20th century, I expect, where arguably the brakes were applied to pure unbirdled capitalism in the name of workers' rights and -- if anything -- probably retarded employment in the short term.
---Joel